Encoder-side search ranges having horizontal bias or vertical bias

ABSTRACT

Innovations in encoder-side search ranges having horizontal bias or vertical bias are described herein. For example, a video encoder determines a block vector (“BV”) for a current block of a picture, performs intra prediction for the current block using the BV, and encodes the BV. The BV indicates a displacement to a region within the picture. When determining the BV, the encoder checks a constraint that the region is within a BV search range having a horizontal bias or vertical bias. The encoder can select the BV search range from among multiple available BV search ranges, e.g., depending at least in part on BV values of one or more previous blocks, which can be tracked in a histogram data structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 17/581,446, filed Jan. 21, 2022, which is a continuation ofU.S. patent application Ser. No. 14/455,856, filed Aug. 8, 2014, thedisclosure of which is hereby incorporated by reference. U.S. patentapplication Ser. No. 14/455,856 claims the benefit of U.S. ProvisionalPatent Application No. 61/928,970, filed Jan. 17, 2014, the disclosureof which is hereby incorporated by reference. U.S. patent applicationSer. No. 14/455,856 also claims the benefit of U.S. Provisional PatentApplication No. 61/954,572, filed Mar. 17, 2014, the disclosure of whichis hereby incorporated by reference.

BACKGROUND

Engineers use compression (also called source coding or source encoding)to reduce the bit rate of digital video. Compression decreases the costof storing and transmitting video information by converting theinformation into a lower bit rate form. Decompression (also calleddecoding) reconstructs a version of the original information from thecompressed form. A “codec” is an encoder/decoder system.

Over the last two decades, various video codec standards have beenadopted, including the ITU-T H.261, H.262 (MPEG-2 or ISO/IEC 13818-2),H.263 and H.264 (MPEG-4 AVC or ISO/IEC 14496-10) standards, the MPEG-1(ISO/IEC 11172-2) and MPEG-4 Visual (ISO/IEC 14496-2) standards, and theSMPTE 421M (VC-1) standard. More recently, the H.265/HEVC standard(ITU-T H.265 or ISO/IEC 23008-2) has been approved. Extensions to theH.265/HEVC standard (e.g., for scalable video coding/decoding, forcoding/decoding of video with higher fidelity in terms of sample bitdepth or chroma sampling rate, for screen capture content, or formulti-view coding/decoding) are currently under development. A videocodec standard typically defines options for the syntax of an encodedvideo bitstream, detailing parameters in the bitstream when particularfeatures are used in encoding and decoding. In many cases, a video codecstandard also provides details about the decoding operations a decodershould perform to achieve conforming results in decoding. Aside fromcodec standards, various proprietary codec formats define other optionsfor the syntax of an encoded video bitstream and corresponding decodingoperations.

Intra block copy (“BC”) is a prediction mode under development for HEVCextensions. For intra BC prediction mode, the sample values of a currentblock of a picture are predicted using previously reconstructed samplevalues in the same picture. A block vector (“BV”) indicates adisplacement from the current block to a region of the picture thatincludes the previously reconstructed sample values used for prediction.The BV is signaled in the bitstream. Intra BC prediction is a form ofintra-picture prediction—intra BC prediction for a block of a picturedoes not use any sample values other than sample values in the samepicture.

As currently specified in the HEVC standard and implemented in somereference software for the HEVC standard, intra BC prediction mode hasseveral problems. In particular, options for blocks sizes for intra BCprediction are too limited in many scenarios, and encoder-side decisionsabout block sizes and how to use intra BC prediction are not madeefficiently in many scenarios.

SUMMARY

In summary, the detailed description presents innovations in intra blockcopy (“BC”) prediction as well as innovations in encoder-side searchpatterns, search ranges and approaches to partitioning. For example,some of the innovations relate to use of asymmetric partitions(sometimes called “AMP”) for intra BC prediction. Other innovationsrelate to search patterns or approaches that an encoder uses duringblock vector (“BV”) estimation (for intra BC prediction) or motionestimation. Still other innovations relate to uses of BV search rangesthat have a horizontal or vertical bias during BV estimation.

According to a first aspect of the innovations described herein, animage encoder or video encoder encodes an image or video to produceencoded data, and outputs the encoded data as part of a bitstream. Aspart of the encoding, the encoder performs intra BC prediction for acurrent block that is asymmetrically partitioned for the intra BCprediction. For example, the current block is a 2N×2N block, and thecurrent block is partitioned into (1) a 2N×N/2 block and 2N×3N/2 blockor (2) a 2N×3N/2 block and 2N×N/2 block. Or, as another example, thecurrent block is a 2N×2N block, and the current block is partitionedinto (1) an N/2×2N block and 3N/2×2N block or (2) a 3N/2×2N block andN/2×2N block. More generally, for asymmetric partitioning, the currentblock can be split into two partitions that have different dimensions.As part of the encoding, the encoder can also perform intra BCprediction for another block that is symmetrically partitioned for theintra BC prediction. For example, the other block is a 2N×2N block thatis partitioned into (1) two 2N×N blocks, (2) two N×2N blocks, or (3)four N×N blocks, each of which can be further partitioned into two N×N/2blocks, two N/2×N blocks or four N/2×N/2 blocks. More generally, forsymmetric partitioning, the other block can be split into partitionsthat have identical dimensions.

According to a second aspect of the innovations described herein, animage decoder or video decoder receives encoded data as part of abitstream and decodes the encoded data to reconstruct an image or video.As part of the decoding, the decoder performs intra BC prediction for acurrent block that is asymmetrically partitioned for the intra BCprediction. For example, the current block is a 2N×2N block, and thecurrent block is partitioned into (1) a 2N×N/2 block and 2N×3N/2 blockor (2) a 2N×3N/2 block and 2N×N/2 block. Or, as another example, thecurrent block is a 2N×2N block, and the current block is partitionedinto (1) an N/2×2N block and 3N/2×2N block or (2) a 3N/2×2N block andN/2×2N block. More generally, for the asymmetric partitioning, thecurrent block can be split into two partitions that have differentdimensions. As part of the decoding, the decoder can also perform intraBC prediction for another block that is symmetrically partitioned forthe intra BC prediction. For example, the other block is a 2N×2N blockthat is partitioned into (1) two 2N×N blocks, (2) two N×2N blocks, or(3) four N×N blocks, each of which can be further partitioned into twoN×N/2 blocks, two N/2×N blocks or four N/2×N/2 blocks. More generally,for symmetric partitioning, the other block can be split into partitionsthat have identical dimensions.

According to a third aspect of the innovations described herein, animage encoder or video encoder encodes an image or video to produceencoded data, and outputs the encoded data as part of a bitstream. Aspart of the encoding, the encoder computes a prediction for a currentblock (e.g., prediction block of a prediction unit) of a currentpicture. The prediction can be for motion estimation or BV estimationfor intra BC prediction. In any case, the computing the prediction usesa bottom-up approach to identify partitions of the current block. Ingeneral, the partitions for the current block include two or morepartitions that have different dimensions. For example, the currentblock is a 2N×2N block, and the bottom-up approach includes: (a)checking modes per N×N block of the 2N×2N block; (b) selecting bestmodes for the respective N×N blocks; (c) caching vector values for therespective N×N blocks; (d) checking modes with a 2N-dimension for the2N×2N block, including using the cached vector values; (e) selecting abest mode with a 2N-dimension for the 2N×2N block; and (f) selectingbetween the best mode with a 2N-dimension for the 2N×2N block and theselected best modes for the respective N×N blocks of the 2N×2N block.Or, as another example, the current block is a 2N×2N block, and thebottom-up approach includes: (a) checking a subset of modes per N×Nblock of the 2N×2N block; (b) caching vector values for the respectiveN×N blocks; (c) checking a subset of modes with a 2N-dimension for the2N×2N block, including using the cached vector values; (d) selecting abest mode with a 2N-dimension for the 2N×2N block; and (e) selectingbetween the best mode with a 2N-dimension for the 2N×2N block and bestmodes for the respective N×N blocks.

According to a fourth aspect of the innovations described herein, animage encoder or video encoder encodes an image or video to produceencoded data, and outputs the encoded data as part of a bitstream. Aspart of the encoding, the encoder computes a prediction for a currentblock of a current picture. The prediction can be for motion estimationor BV estimation for intra BC prediction. In any case, the computing theprediction includes (a) identifying a current best location for theprediction through iterative evaluation in a small neighborhood (e.g.,locations that are immediately adjacent horizontally or vertically tothe current best location) around the current best location; and (b)confirming the current best location through iterative evaluation insuccessively larger neighborhoods (e.g., locations in rings outside thesmall neighborhood) around the current best location. For example, ifthe current best location is worse than a location in one of the largerneighborhoods, the encoder replaces the current best location andrepeats the identifying and the confirming. The confirming stage canstop if a threshold number of iterations of evaluation in successivelylarger neighborhoods is reached.

According to a fifth aspect of the innovations described herein, animage encoder or video encoder determines a BV for a current block of apicture, performs intra BC prediction for the current block using theBV, and encodes the BV. The BV indicates a displacement to a regionwithin the picture. When determining the BV, the encoder checks aconstraint that the region is within a BV search range having ahorizontal bias or vertical bias. The encoder can select the BV searchrange from among multiple available BV search ranges, e.g., depending atleast in part on BV values of one or more previous blocks, which can betracked in a histogram data structure.

According to a sixth aspect of the innovations described herein, animage encoder or video encoder encodes data for a picture using intra BCprediction, and outputs the encoded data as part of a bitstream. As partof the encoding, the encoder performs BV estimation operations using aBV search range with a horizontal or vertical bias. The encoder canselect the BV search range from among multiple available BV searchranges, e.g., depending at least in part on BV values of one or moreprevious blocks, which can be tracked in a histogram data structure.

The innovations can be implemented as part of a method, as part of acomputing device adapted to perform the method or as part of a tangiblecomputer-readable media storing computer-executable instructions forcausing a computing device to perform the method. The variousinnovations can be used in combination or separately.

The foregoing and other objects, features, and advantages of theinvention will become more apparent from the following detaileddescription, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an example computing system in which somedescribed embodiments can be implemented.

FIGS. 2 a and 2 b are diagrams of example network environments in whichsome described embodiments can be implemented.

FIG. 3 is a diagram of an example encoder system in conjunction withwhich some described embodiments can be implemented.

FIG. 4 is a diagram of an example decoder system in conjunction withwhich some described embodiments can be implemented.

FIGS. 5 a and 5 b are diagrams illustrating an example video encoder inconjunction with which some described embodiments can be implemented.

FIG. 6 is a diagram illustrating an example video decoder in conjunctionwith which some described embodiments can be implemented.

FIGS. 7 and 8 are diagrams illustrating intra BC prediction for a blockof a picture and candidate blocks for the block in block matching.

FIG. 9 is a diagram illustrating example block sizes for intra BCprediction, including some asymmetric partitions and some symmetricpartitions.

FIGS. 10 and 11 are generalized techniques for encoding and decoding,respectively, that include intra BC prediction with asymmetricpartitions.

FIG. 12 is a diagram illustrating motion estimation for a block of apicture.

FIG. 13 is a flowchart and accompanying diagram illustrating a top-downapproach to partitioning an intra-BC-predicted block.

FIG. 14 is a flowchart illustrating a generalized technique for using abottom-up approach to partitioning.

FIG. 15 is a flowchart and accompanying diagram illustrating a bottom-upapproach to partitioning an intra-BC-predicted block.

FIG. 16 is a flowchart and accompanying diagram illustrating even fasterbottom-up approaches to partitioning an intra-BC-predicted block.

FIG. 17 is a flowchart illustrating a generalized technique forsearching for a BV value or MV value for a block using iterativeevaluation of a location in small neighborhood(s) and iterativeconfirmation of the location in larger neighborhood(s).

FIGS. 18 a and 18 b are diagrams illustrating iterative evaluation of alocation in a small neighborhood, when searching for a BV value or MVvalue for a block.

FIG. 19 is a diagram illustrating iterative confirmation of a locationin one or more larger neighborhoods, when searching for a BV value or MVvalue for a block.

FIG. 20 is a flowchart illustrating an example technique for searchingfor a BV value or MV value for a block using iterative evaluation of alocation in small neighborhood(s) and iterative confirmation of thelocation in larger neighborhood(s).

FIGS. 21 a-21 e are diagrams illustrating example constraints on searchrange for BV values.

FIG. 22 is a flowchart illustrating a generalized technique for encodingwith an intra BC prediction mode, subject to one or more constraints onselection of BV values.

FIG. 23 is a diagram illustrating example z-scan order for blocks of apicture.

DETAILED DESCRIPTION

The detailed description presents innovations in intra block copy (“BC”)prediction as well as innovations in encoder-side search patterns,search ranges and approaches to partitioning. For example, some of theinnovations relate to use of asymmetric partitions (sometimes called“AMP”) for intra BC prediction during encoding and/or decoding. Otherinnovations relate to search patterns or approaches that an encoder usesduring block vector (“BV”) estimation (for intra BC prediction) ormotion estimation. Still other innovations relate to uses of BV searchranges that have a horizontal or vertical bias during BV estimation.

Although operations described herein are in places described as beingperformed by a video encoder or video decoder, in many cases theoperations can be performed by another type of media processing tool(e.g., image encoder or image decoder).

Some of the innovations described herein are illustrated with referenceto syntax elements and operations specific to the H.265/HEVC standard.For example, reference is made to the draft version JCTVC-P1005 of theH.265/HEVC standard—“High Efficiency Video Coding (HEVC) RangeExtensions Text Specification: Draft 6,” JCTVC-P1005_v1, February 2014.The innovations described herein can also be implemented for otherstandards or formats.

Many of the innovations described herein can improve rate-distortionperformance when encoding certain “artificially-created” video contentsuch as screen capture content. In general, screen capture video (alsocalled screen content video) is video that contains rendered text,computer graphics, animation-generated content or other similar types ofcontent captured when rendered to a computer display, as opposed tocamera-captured video content only. Screen capture content typicallyincludes repeated structures (e.g., graphics, text characters). Screencapture content is usually encoded in a format (e.g., YUV 4:4:4 or RGB4:4:4) with high chroma sampling resolution, although it may also beencoded in a format with lower chroma sampling resolution (e.g., YUV4:2:0). Common scenarios for encoding/decoding of screen capture contentinclude remote desktop conferencing and encoding/decoding of graphicaloverlays on natural video or other “mixed content” video. Several of theinnovations described herein are adapted for encoding of screen contentvideo or other artificially-created video. These innovations can also beused for natural video, but may not be as effective. Other innovationsdescribed herein are effective in encoding of natural video orartificially-created video.

More generally, various alternatives to the examples described hereinare possible. For example, some of the methods described herein can bealtered by changing the ordering of the method acts described, bysplitting, repeating, or omitting certain method acts, etc. The variousaspects of the disclosed technology can be used in combination orseparately. Different embodiments use one or more of the describedinnovations. Some of the innovations described herein address one ormore of the problems noted in the background. Typically, a giventechnique/tool does not solve all such problems.

I. EXAMPLE COMPUTING SYSTEMS

FIG. 1 illustrates a generalized example of a suitable computing system(100) in which several of the described innovations may be implemented.The computing system (100) is not intended to suggest any limitation asto scope of use or functionality, as the innovations may be implementedin diverse general-purpose or special-purpose computing systems.

With reference to FIG. 1 , the computing system (100) includes one ormore processing units (110, 115) and memory (120, 125). The processingunits (110, 115) execute computer-executable instructions. A processingunit can be a general-purpose central processing unit (“CPU”), processorin an application-specific integrated circuit (“ASIC”) or any other typeof processor. In a multi-processing system, multiple processing unitsexecute computer-executable instructions to increase processing power.For example, FIG. 1 shows a central processing unit (110) as well as agraphics processing unit or co-processing unit (115). The tangiblememory (120, 125) may be volatile memory (e.g., registers, cache, RAM),non-volatile memory (e.g., ROM, EEPROM, flash memory, etc.), or somecombination of the two, accessible by the processing unit(s). The memory(120, 125) stores software (180) implementing one or more innovationsfor intra BC prediction with asymmetric partitions and/or one or moreinnovations for encoder-side search patterns, search ranges having ahorizontal or vertical bias and/or approaches to partitioning, in theform of computer-executable instructions suitable for execution by theprocessing unit(s).

A computing system may have additional features. For example, thecomputing system (100) includes storage (140), one or more input devices(150), one or more output devices (160), and one or more communicationconnections (170). An interconnection mechanism (not shown) such as abus, controller, or network interconnects the components of thecomputing system (100). Typically, operating system software (not shown)provides an operating environment for other software executing in thecomputing system (100), and coordinates activities of the components ofthe computing system (100).

The tangible storage (140) may be removable or non-removable, andincludes magnetic disks, magnetic tapes or cassettes, CD-ROMs, DVDs, orany other medium which can be used to store information and which can beaccessed within the computing system (100). The storage (140) storesinstructions for the software (180) implementing one or more innovationsfor intra BC prediction with asymmetric partitions and/or one or moreinnovations for encoder-side search patterns, search ranges and/orapproaches to partitioning.

The input device(s) (150) may be a touch input device such as akeyboard, mouse, pen, or trackball, a voice input device, a scanningdevice, or another device that provides input to the computing system(100). For video, the input device(s) (150) may be a camera, video card,TV tuner card, screen capture module, or similar device that acceptsvideo input in analog or digital form, or a CD-ROM or CD-RW that readsvideo input into the computing system (100). The output device(s) (160)may be a display, printer, speaker, CD-writer, or another device thatprovides output from the computing system (100).

The communication connection(s) (170) enable communication over acommunication medium to another computing entity. The communicationmedium conveys information such as computer-executable instructions,audio or video input or output, or other data in a modulated datasignal. A modulated data signal is a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia can use an electrical, optical, RF, or other carrier.

The innovations can be described in the general context ofcomputer-readable media. Computer-readable media are any availabletangible media that can be accessed within a computing environment. Byway of example, and not limitation, with the computing system (100),computer-readable media include memory (120, 125), storage (140), andcombinations of any of the above.

The innovations can be described in the general context ofcomputer-executable instructions, such as those included in programmodules, being executed in a computing system on a target real orvirtual processor. Generally, program modules include routines,programs, libraries, objects, classes, components, data structures, etc.that perform particular tasks or implement particular abstract datatypes. The functionality of the program modules may be combined or splitbetween program modules as desired in various embodiments.Computer-executable instructions for program modules may be executedwithin a local or distributed computing system.

The terms “system” and “device” are used interchangeably herein. Unlessthe context clearly indicates otherwise, neither term implies anylimitation on a type of computing system or computing device. Ingeneral, a computing system or computing device can be local ordistributed, and can include any combination of special-purpose hardwareand/or general-purpose hardware with software implementing thefunctionality described herein.

The disclosed methods can also be implemented using specializedcomputing hardware configured to perform any of the disclosed methods.For example, the disclosed methods can be implemented by an integratedcircuit (e.g., an ASIC (such as an ASIC digital signal processor(“DSP”), a graphics processing unit (“GPU”), or a programmable logicdevice (“PLD”), such as a field programmable gate array (“FPGA”))specially designed or configured to implement any of the disclosedmethods.

For the sake of presentation, the detailed description uses terms like“determine” and “use” to describe computer operations in a computingsystem. These terms are high-level abstractions for operations performedby a computer, and should not be confused with acts performed by a humanbeing. The actual computer operations corresponding to these terms varydepending on implementation. As used herein to describe a coding option,the term “best” (as in “best location,” “best mode” for partitioning or“best combination”) indicates a preferred coding option, compared toother coding options, with respect to estimated coding efficiency oractual coding efficiency, in terms of distortion cost, bit rate cost orsome combination of distortion cost and bit rate cost. Any availabledistortion metric can be used for distortion cost. Any available bitrate metric can be used for bit rate cost. Other factors (such asalgorithmic coding complexity, algorithmic decoding complexity, resourceusage and/or delay) can also affect the decision about which codingoption is “best.”

II. EXAMPLE NETWORK ENVIRONMENTS

FIGS. 2 a and 2 b show example network environments (201, 202) thatinclude video encoders (220) and video decoders (270). The encoders(220) and decoders (270) are connected over a network (250) using anappropriate communication protocol. The network (250) can include theInternet or another computer network.

In the network environment (201) shown in FIG. 2 a , each real-timecommunication (“RTC”) tool (210) includes both an encoder (220) and adecoder (270) for bidirectional communication. A given encoder (220) canproduce output compliant with a variation or extension of the H.265/HEVCstandard, SMPTE 421M standard, ISO/IEC 14496-10 standard (also known asH.264 or AVC), another standard, or a proprietary format, with acorresponding decoder (270) accepting encoded data from the encoder(220). The bidirectional communication can be part of a videoconference, video telephone call, or other two-party or multi-partcommunication scenario. Although the network environment (201) in FIG. 2a includes two real-time communication tools (210), the networkenvironment (201) can instead include three or more real-timecommunication tools (210) that participate in multi-party communication.

A real-time communication tool (210) manages encoding by an encoder(220). FIG. 3 shows an example encoder system (300) that can be includedin the real-time communication tool (210). Alternatively, the real-timecommunication tool (210) uses another encoder system. A real-timecommunication tool (210) also manages decoding by a decoder (270). FIG.4 shows an example decoder system (400), which can be included in thereal-time communication tool (210). Alternatively, the real-timecommunication tool (210) uses another decoder system.

In the network environment (202) shown in FIG. 2 b , an encoding tool(212) includes an encoder (220) that encodes video for delivery tomultiple playback tools (214), which include decoders (270). Theunidirectional communication can be provided for a video surveillancesystem, web camera monitoring system, screen capture module, remotedesktop conferencing presentation or other scenario in which video isencoded and sent from one location to one or more other locations.Although the network environment (202) in FIG. 2 b includes two playbacktools (214), the network environment (202) can include more or fewerplayback tools (214). In general, a playback tool (214) communicateswith the encoding tool (212) to determine a stream of video for theplayback tool (214) to receive. The playback tool (214) receives thestream, buffers the received encoded data for an appropriate period, andbegins decoding and playback.

FIG. 3 shows an example encoder system (300) that can be included in theencoding tool (212). Alternatively, the encoding tool (212) uses anotherencoder system. The encoding tool (212) can also include server-sidecontroller logic for managing connections with one or more playbacktools (214). FIG. 4 shows an example decoder system (400), which can beincluded in the playback tool (214). Alternatively, the playback tool(214) uses another decoder system. A playback tool (214) can alsoinclude client-side controller logic for managing connections with theencoding tool (212).

III. EXAMPLE ENCODER SYSTEMS

FIG. 3 is a block diagram of an example encoder system (300) inconjunction with which some described embodiments may be implemented.The encoder system (300) can be a general-purpose encoding tool capableof operating in any of multiple encoding modes such as a low-latencyencoding mode for real-time communication, a transcoding mode, and ahigher-latency encoding mode for producing media for playback from afile or stream, or it can be a special-purpose encoding tool adapted forone such encoding mode. The encoder system (300) can be adapted forencoding of a particular type of content (e.g., screen capture content).The encoder system (300) can be implemented as an operating systemmodule, as part of an application library or as a standaloneapplication. Overall, the encoder system (300) receives a sequence ofsource video frames (311) from a video source (310) and produces encodeddata as output to a channel (390). The encoded data output to thechannel can include content encoded using intra BC prediction mode.

The video source (310) can be a camera, tuner card, storage media,screen capture module, or other digital video source. The video source(310) produces a sequence of video frames at a frame rate of, forexample, 30 frames per second. As used herein, the term “frame”generally refers to source, coded or reconstructed image data. Forprogressive-scan video, a frame is a progressive-scan video frame. Forinterlaced video, in example embodiments, an interlaced video framemight be de-interlaced prior to encoding. Alternatively, twocomplementary interlaced video fields are encoded together as a singlevideo frame or encoded as two separately-encoded fields. Aside fromindicating a progressive-scan video frame or interlaced-scan videoframe, the term “frame” or “picture” can indicate a single non-pairedvideo field, a complementary pair of video fields, a video object planethat represents a video object at a given time, or a region of interestin a larger image. The video object plane or region can be part of alarger image that includes multiple objects or regions of a scene.

An arriving source frame (311) is stored in a source frame temporarymemory storage area (320) that includes multiple frame buffer storageareas (321, 322, . . . , 32 n). A frame buffer (321, 322, etc.) holdsone source frame in the source frame storage area (320). After one ormore of the source frames (311) have been stored in frame buffers (321,322, etc.), a frame selector (330) selects an individual source framefrom the source frame storage area (320). The order in which frames areselected by the frame selector (330) for input to the encoder (340) maydiffer from the order in which the frames are produced by the videosource (310), e.g., the encoding of some frames may be delayed in order,so as to allow some later frames to be encoded first and to thusfacilitate temporally backward prediction. Before the encoder (340), theencoder system (300) can include a pre-processor (not shown) thatperforms pre-processing (e.g., filtering) of the selected frame (331)before encoding. The pre-processing can include color space conversioninto primary (e.g., luma) and secondary (e.g., chroma differences towardred and toward blue) components and resampling processing (e.g., toreduce the spatial resolution of chroma components) for encoding.Typically, before encoding, video has been converted to a color spacesuch as YUV, in which sample values of a luma (Y) component representbrightness or intensity values, and sample values of chroma (U, V)components represent color-difference values. The precise definitions ofthe color-difference values (and conversion operations to/from YUV colorspace to another color space such as RGB) depend on implementation. Ingeneral, as used herein, the term YUV indicates any color space with aluma (or luminance) component and one or more chroma (or chrominance)components, including Y′UV, YIQ, Y′IQ and YDbDr as well as variationssuch as YCbCr and YCoCg. The chroma sample values may be sub-sampled toa lower chroma sampling rate (e.g., for YUV 4:2:0 format), or the chromasample values may have the same resolution as the luma sample values(e.g., for YUV 4:4:4 format). Or, the video can be encoded in anotherformat (e.g., RGB 4:4:4 format, GBR 4:4:4 format or BGR 4:4:4 format).

The encoder (340) encodes the selected frame (331) to produce a codedframe (341) and also produces memory management control operation(“MMCO”) signals (342) or reference picture set (“RPS”) information. TheRPS is the set of frames that may be used for reference in motioncompensation for a current frame or any subsequent frame. If the currentframe is not the first frame that has been encoded, when performing itsencoding process, the encoder (340) may use one or more previouslyencoded/decoded frames (369) that have been stored in a decoded frametemporary memory storage area (360). Such stored decoded frames (369)are used as reference frames for inter-frame prediction of the contentof the current source frame (331). The MMCO/RPS information (342)indicates to a decoder which reconstructed frames may be used asreference frames, and hence should be stored in a frame storage area.

Generally, the encoder (340) includes multiple encoding modules thatperform encoding tasks such as partitioning into tiles, intra predictionestimation and prediction, motion estimation and compensation, frequencytransforms, quantization and entropy coding. The exact operationsperformed by the encoder (340) can vary depending on compression format.The format of the output encoded data can be a variation or extension ofH.265/HEVC format, Windows Media Video format, VC-1 format, MPEG-xformat (e.g., MPEG-1, MPEG-2, or MPEG-4), H.26x format (e.g., H.261,H.262, H.263, H.264), or another format.

The encoder (340) can partition a frame into multiple tiles of the samesize or different sizes. For example, the encoder (340) splits the framealong tile rows and tile columns that, with frame boundaries, definehorizontal and vertical boundaries of tiles within the frame, where eachtile is a rectangular region. Tiles are often used to provide optionsfor parallel processing. A frame can also be organized as one or moreslices, where a slice can be an entire frame or region of the frame. Aslice can be decoded independently of other slices in a frame, whichimproves error resilience. The content of a slice or tile is furtherpartitioned into blocks or other sets of sample values for purposes ofencoding and decoding.

For syntax according to the H.265/HEVC standard, the encoder splits thecontent of a frame (or slice or tile) into coding tree units. A codingtree unit (“CTU”) includes luma sample values organized as a luma codingtree block (“CTB”) and corresponding chroma sample values organized astwo chroma CTBs. The size of a CTU (and its CTBs) is selected by theencoder. A luma CTB can contain, for example, 64×64, 32×32 or 16×16 lumasample values. A CTU includes one or more coding units. A coding unit(“CU”) has a luma coding block (“CB”) and two corresponding chroma CBs.For example, a CTU with a 64×64 luma CTB and two 64×64 chroma CTBs (YUV4:4:4 format) can be split into four CUs, with each CU including a 32×32luma CB and two 32×32 chroma CBs, and with each CU possibly being splitfurther into smaller CUs. Or, as another example, a CTU with a 64×64luma CTB and two 32×32 chroma CTBs (YUV 4:2:0 format) can be split intofour CUs, with each CU including a 32×32 luma CB and two 16×16 chromaCBs, and with each CU possibly being split further into smaller CUs. Thesmallest allowable size of CU (e.g., 8×8, 16×16) can be signaled in thebitstream.

Generally, a CU has a prediction mode such as inter or intra. A CUincludes one or more prediction units for purposes of signaling ofprediction information (such as prediction mode details, displacementvalues, etc.) and/or prediction processing. A prediction unit (“PU”) hasa luma prediction block (“PB”) and two chroma PBs. According to theH.265/HEVC standard, for an intra-predicted CU, the PU has the same sizeas the CU, unless the CU has the smallest size (e.g., 8×8). In thatcase, the CU can be split into four smaller PUs (e.g., each 4×4 if thesmallest CU size is 8×8, for intra prediction) or the PU can have thesmallest CU size, as indicated by a syntax element for the CU. Forasymmetric partitions used in intra BC prediction, however, a CU can besplit into multiple PUs as shown in FIG. 9 . In this case, a larger CU(e.g., 64×64, 32×32 or 16×16) or CU of the smallest size (e.g., 8×8) canbe split into multiple PUs.

A CU also has one or more transform units for purposes of residualcoding/decoding, where a transform unit (“TU”) has a luma transformblock (“TB”) and two chroma TBs. A PU in an intra-predicted CU maycontain a single TU (equal in size to the PU) or multiple TUs. Theencoder decides how to partition video into CTUs, CUs, PUs, TUs, etc.

In H.265/HEVC implementations, a slice can include a single slicesegment (independent slice segment) or be divided into multiple slicesegments (independent slice segment and one or more dependent slicesegments). A slice segment is an integer number of CTUs orderedconsecutively in a tile scan, contained in a single network abstractionlayer (“NAL”) unit. For an independent slice segment, a slice segmentheader includes values of syntax elements that apply for the independentslice segment. For a dependent slice segment, a truncated slice segmentheader includes a few values of syntax elements that apply for thatdependent slice segment, and the values of the other syntax elements forthe dependent slice segment are inferred from the values for thepreceding independent slice segment in decoding order.

As used herein, the term “block” can indicate a macroblock, predictionunit, residual data unit, or a CB, PB or TB, or some other set of samplevalues, depending on context.

Returning to FIG. 3 , the encoder represents an intra-coded block of asource frame (331) in terms of prediction from other, previouslyreconstructed sample values in the frame (331). For intra BC prediction,an intra-picture estimator estimates displacement of a block withrespect to the other, previously reconstructed sample values. Anintra-frame prediction reference region is a region of sample values inthe frame that are used to generate BC-prediction values for the block.The intra-frame prediction region can be indicated with a block vector(“BV”) value (determined in BV estimation). Example approaches to makingdecisions during intra-picture encoding are described below. Dependingon implementation, the encoder can perform BV estimation for a blockusing input sample values or reconstructed sample values (previouslyencoded sample values in the same picture). For additional details, seethe description of BV estimation in section V.

For intra spatial prediction for a block, the intra-picture estimatorestimates extrapolation of the neighboring reconstructed sample valuesinto the block. The intra-picture estimator can output predictioninformation (such as BV values for intra BC prediction, or predictionmode (direction) for intra spatial prediction), which is entropy coded.An intra-frame prediction predictor applies the prediction informationto determine intra prediction values.

The encoder (340) represents an inter-frame coded, predicted block of asource frame (331) in terms of prediction from reference frames. Amotion estimator estimates the motion of the block with respect to oneor more reference frames (369). When multiple reference frames are used,the multiple reference frames can be from different temporal directionsor the same temporal direction. A motion-compensated predictionreference region is a region of sample values in the reference frame(s)that are used to generate motion-compensated prediction values for ablock of sample values of a current frame. The motion estimator outputsmotion information such as motion vector (“MV”) information, which isentropy coded. A motion compensator applies MVs to reference frames(369) to determine motion-compensated prediction values for inter-frameprediction. Example approaches to making decisions during inter-pictureencoding are described below.

The encoder can determine the differences (if any) between a block'sprediction values (intra or inter) and corresponding original values.These prediction residual values are further encoded using a frequencytransform (if the frequency transform is not skipped), quantization andentropy encoding. For example, the encoder (340) sets values forquantization parameter (“QP”) for a picture, tile, slice and/or otherportion of video, and quantizes transform coefficients accordingly. Theentropy coder of the encoder (340) compresses quantized transformcoefficient values as well as certain side information (e.g., MVinformation, index values for BV predictors, BV differentials, QPvalues, mode decisions, parameter choices). Typical entropy codingtechniques include Exponential-Golomb coding, Golomb-Rice coding,arithmetic coding, differential coding, Huffman coding, run lengthcoding, variable-length-to-variable-length (“V2V”) coding,variable-length-to-fixed-length (“V2F”) coding, Lempel-Ziv (“LZ”)coding, dictionary coding, probability interval partitioning entropycoding (“PIPE”), and combinations of the above. The entropy coder canuse different coding techniques for different kinds of information, canapply multiple techniques in combination (e.g., by applying Golomb-Ricecoding followed by arithmetic coding), and can choose from amongmultiple code tables within a particular coding technique. In someimplementations, the frequency transform can be skipped. In this case,prediction residual values can be quantized and entropy coded.

An adaptive deblocking filter is included within the motion compensationloop (that is, “in-loop” filtering) in the encoder (340) to smoothdiscontinuities across block boundary rows and/or columns in a decodedframe. Other filtering (such as de-ringing filtering, adaptive loopfiltering (“ALF”), or sample-adaptive offset (“SAO”) filtering; notshown) can alternatively or additionally be applied as in-loop filteringoperations.

The encoded data produced by the encoder (340) includes syntax elementsfor various layers of bitstream syntax. For syntax according to theH.265/HEVC standard, for example, a picture parameter set (“PPS”) is asyntax structure that contains syntax elements that may be associatedwith a picture. A PPS can be used for a single picture, or a PPS can bereused for multiple pictures in a sequence. A PPS is typically signaledseparate from encoded data for a picture (e.g., one NAL unit for a PPS,and one or more other NAL units for encoded data for a picture). Withinthe encoded data for a picture, a syntax element indicates which PPS touse for the picture. Similarly, for syntax according to the H.265/HEVCstandard, a sequence parameter set (“SPS”) is a syntax structure thatcontains syntax elements that may be associated with a sequence ofpictures. A bitstream can include a single SPS or multiple SPSs. A SPSis typically signaled separate from other data for the sequence, and asyntax element in the other data indicates which SPS to use.

The coded frames (341) and MMCO/RPS information (342) (or informationequivalent to the MMCO/RPS information (342), since the dependencies andordering structures for frames are already known at the encoder (340))are processed by a decoding process emulator (350). The decoding processemulator (350) implements some of the functionality of a decoder, forexample, decoding tasks to reconstruct reference frames. In a mannerconsistent with the MMCO/RPS information (342), the decoding processesemulator (350) determines whether a given coded frame (341) needs to bereconstructed and stored for use as a reference frame in inter-frameprediction of subsequent frames to be encoded. If a coded frame (341)needs to be stored, the decoding process emulator (350) models thedecoding process that would be conducted by a decoder that receives thecoded frame (341) and produces a corresponding decoded frame (351). Indoing so, when the encoder (340) has used decoded frame(s) (369) thathave been stored in the decoded frame storage area (360), the decodingprocess emulator (350) also uses the decoded frame(s) (369) from thestorage area (360) as part of the decoding process.

The decoded frame temporary memory storage area (360) includes multipleframe buffer storage areas (361, 362, . . . , 36 n). In a mannerconsistent with the MMCO/RPS information (342), the decoding processemulator (350) manages the contents of the storage area (360) in orderto identify any frame buffers (361, 362, etc.) with frames that are nolonger needed by the encoder (340) for use as reference frames. Aftermodeling the decoding process, the decoding process emulator (350)stores a newly decoded frame (351) in a frame buffer (361, 362, etc.)that has been identified in this manner.

The coded frames (341) and MMCO/RPS information (342) are buffered in atemporary coded data area (370). The coded data that is aggregated inthe coded data area (370) contains, as part of the syntax of anelementary coded video bitstream, encoded data for one or more pictures.The coded data that is aggregated in the coded data area (370) can alsoinclude media metadata relating to the coded video data (e.g., as one ormore parameters in one or more supplemental enhancement information(“SEI”) messages or video usability information (“VUI”) messages).

The aggregated data (371) from the temporary coded data area (370) areprocessed by a channel encoder (380). The channel encoder (380) canpacketize and/or multiplex the aggregated data for transmission orstorage as a media stream (e.g., according to a media program stream ortransport stream format such as ITU-T H.222.0|ISO/IEC 13818-1 or anInternet real-time transport protocol format such as IETF RFC 3550), inwhich case the channel encoder (380) can add syntax elements as part ofthe syntax of the media transmission stream. Or, the channel encoder(380) can organize the aggregated data for storage as a file (e.g.,according to a media container format such as ISO/IEC 14496-12), inwhich case the channel encoder (380) can add syntax elements as part ofthe syntax of the media storage file. Or, more generally, the channelencoder (380) can implement one or more media system multiplexingprotocols or transport protocols, in which case the channel encoder(380) can add syntax elements as part of the syntax of the protocol(s).The channel encoder (380) provides output to a channel (390), whichrepresents storage, a communications connection, or another channel forthe output. The channel encoder (380) or channel (390) may also includeother elements (not shown), e.g., for forward-error correction (“FEC”)encoding and analog signal modulation.

IV. EXAMPLE DECODER SYSTEMS

FIG. 4 is a block diagram of an example decoder system (400) inconjunction with which some described embodiments may be implemented.The decoder system (400) can be a general-purpose decoding tool capableof operating in any of multiple decoding modes such as a low-latencydecoding mode for real-time communication and a higher-latency decodingmode for media playback from a file or stream, or it can be aspecial-purpose decoding tool adapted for one such decoding mode. Thedecoder system (400) can be implemented as an operating system module,as part of an application library or as a standalone application.Overall, the decoder system (400) receives coded data from a channel(410) and produces reconstructed frames as output for an outputdestination (490).

The decoder system (400) includes a channel (410), which can representstorage, a communications connection, or another channel for coded dataas input. The channel (410) produces coded data that has been channelcoded. A channel decoder (420) can process the coded data. For example,the channel decoder (420) de-packetizes and/or demultiplexes data thathas been aggregated for transmission or storage as a media stream (e.g.,according to a media program stream or transport stream format such asITU-T H.222.0|ISO/IEC 13818-1 or an internet real-time transportprotocol format such as IETF RFC 3550), in which case the channeldecoder (420) can parse syntax elements added as part of the syntax ofthe media transmission stream. Or, the channel decoder (420) separatescoded video data that has been aggregated for storage as a file (e.g.,according to a media container format such as ISO/IEC 14496-12), inwhich case the channel decoder (420) can parse syntax elements added aspart of the syntax of the media storage file. Or, more generally, thechannel decoder (420) can implement one or more media systemdemultiplexing protocols or transport protocols, in which case thechannel decoder (420) can parse syntax elements added as part of thesyntax of the protocol(s). The channel (410) or channel decoder (420)may also include other elements (not shown), e.g., for FEC decoding andanalog signal demodulation.

The coded data (421) that is output from the channel decoder (420) isstored in a temporary coded data area (430) until a sufficient quantityof such data has been received. The coded data (421) includes codedframes (431) and MMCO/RPS information (432). The coded data (421) in thecoded data area (430) contain, as part of the syntax of an elementarycoded video bitstream, coded data for one or more pictures. The codeddata (421) in the coded data area (430) can also include media metadatarelating to the encoded video data (e.g., as one or more parameters inone or more SEI messages or VUI messages).

In general, the coded data area (430) temporarily stores coded data(421) until such coded data (421) is used by the decoder (450). At thatpoint, coded data for a coded frame (431) and MMCO/RPS information (432)are transferred from the coded data area (430) to the decoder (450). Asdecoding continues, new coded data is added to the coded data area (430)and the oldest coded data remaining in the coded data area (430) istransferred to the decoder (450).

The decoder (450) decodes a coded frame (431) to produce a correspondingdecoded frame (451). As appropriate, when performing its decodingprocess, the decoder (450) may use one or more previously decoded frames(469) as reference frames for inter-frame prediction. The decoder (450)reads such previously decoded frames (469) from a decoded frametemporary memory storage area (460). Generally, the decoder (450)includes multiple decoding modules that perform decoding tasks such asentropy decoding, intra-frame prediction, motion-compensated inter-frameprediction, inverse quantization, inverse frequency transforms (if notskipped), and merging of tiles. The exact operations performed by thedecoder (450) can vary depending on compression format.

For example, the decoder (450) receives encoded data for a compressedframe or sequence of frames and produces output including decoded frame(451). In the decoder (450), a buffer receives encoded data for acompressed frame and, at an appropriate time, makes the received encodeddata available to an entropy decoder. The entropy decoder entropydecodes entropy-coded quantized data as well as entropy-coded sideinformation, typically applying the inverse of entropy encodingperformed in the encoder. A motion compensator applies motioninformation to one or more reference frames to form motion-compensatedprediction values for any inter-coded blocks of the frame beingreconstructed. An intra-frame prediction module can spatially predictsample values of a current block from neighboring, previouslyreconstructed sample values or, for intra BC prediction, predict samplevalues of a current block using previously reconstructed sample valuesof an intra-frame prediction region in the frame. The intra-frameprediction region can be indicated with a BV value. The decoder (450)also reconstructs prediction residual values. An inverse quantizerinverse quantizes entropy-decoded data. For example, the decoder (450)sets values for QP for a picture, tile, slice and/or other portion ofvideo based on syntax elements in the bitstream, and inverse quantizestransform coefficients accordingly. An inverse frequency transformerconverts the quantized, frequency-domain data into spatial-domain data.In some implementations, the frequency transform can be skipped, inwhich case the inverse frequency transform is also skipped. If so,prediction residual values can be entropy decoded and inverse quantized.For an inter-frame predicted block, the decoder (450) combinesreconstructed prediction residual values with motion-compensatedprediction values. The decoder (450) can similarly combine predictionresidual values with prediction values from intra prediction. Anadaptive deblocking filter is included within the motion compensationloop in the video decoder (450) to smooth discontinuities across blockboundary rows and/or columns in the decoded frame (451). Other filtering(such as de-ringing filtering, ALF, or SAO filtering; not shown) canalternatively or additionally be applied as in-loop filteringoperations.

The decoded frame temporary memory storage area (460) includes multipleframe buffer storage areas (461, 462, . . . , 46 n). The decoded framestorage area (460) is an example of a decoded picture buffer. Thedecoder (450) uses the MMCO/RPS information (432) to identify a framebuffer (461, 462, etc.) in which it can store a decoded frame (451). Thedecoder (450) stores the decoded frame (451) in that frame buffer.

An output sequencer (480) identifies when the next frame to be producedin output order is available in the decoded frame storage area (460).When the next frame (481) to be produced in output order is available inthe decoded frame storage area (460), it is read by the output sequencer(480) and output to the output destination (490) (e.g., display). Ingeneral, the order in which frames are output from the decoded framestorage area (460) by the output sequencer (480) may differ from theorder in which the frames are decoded by the decoder (450).

V. EXAMPLE VIDEO ENCODERS

FIGS. 5 a and 5 b are a block diagram of a generalized video encoder(500) in conjunction with which some described embodiments may beimplemented. The encoder (500) receives a sequence of video picturesincluding a current picture as an input video signal (505) and producesencoded data in a coded video bitstream (595) as output.

The encoder (500) is block-based and uses a block format that depends onimplementation. Blocks may be further sub-divided at different stages,e.g., at the prediction, frequency transform and/or entropy encodingstages. For example, a picture can be divided into 64×64 blocks, 32×32blocks or 16×16 blocks, which can in turn be divided into smaller blocksof sample values for coding and decoding. In implementations of encodingfor the H.265/HEVC standard, the encoder partitions a picture into CTUs(CTBs), CUs (CBs), PUs (PBs) and TU (TBs). Blocks (e.g., CUs) can beasymmetrically partitioned into smaller blocks (e.g., PUs) for purposesof intra BC prediction, as shown in FIG. 9 .

The encoder (500) compresses pictures using intra-picture coding and/orinter-picture coding. Many of the components of the encoder (500) areused for both intra-picture coding and inter-picture coding. The exactoperations performed by those components can vary depending on the typeof information being compressed.

A tiling module (510) optionally partitions a picture into multipletiles of the same size or different sizes. For example, the tilingmodule (510) splits the picture along tile rows and tile columns that,with picture boundaries, define horizontal and vertical boundaries oftiles within the picture, where each tile is a rectangular region. InH.265/HEVC implementations, the encoder (500) partitions a picture intoone or more slices, where each slice includes one or more slicesegments.

The general encoding control (520) receives pictures for the input videosignal (505) as well as feedback (not shown) from various modules of theencoder (500). Overall, the general encoding control (520) providescontrol signals (not shown) to other modules (such as the tiling module(510), transformer/scaler/quantizer (530), scaler/inverse transformer(535), intra-picture estimator (540), motion estimator (550) andintra/inter switch) to set and change coding parameters during encoding.In particular, the general encoding control (520) can manage decisionsabout partitioning during encoding. More generally, the general encodingcontrol (520) can decide whether and how to use intra BC predictionduring encoding. The general encoding control (520) can also evaluateintermediate results during encoding, for example, performingrate-distortion analysis. The general encoding control (520) producesgeneral control data (522) that indicates decisions made duringencoding, so that a corresponding decoder can make consistent decisions.The general control data (522) is provided to the headerformatter/entropy coder (590).

If the current picture is predicted using inter-picture prediction, amotion estimator (550) estimates the motion of blocks of sample valuesof a current picture of the input video signal (505) with respect to oneor more reference pictures. The decoded picture buffer (570) buffers oneor more reconstructed previously coded pictures for use as referencepictures. When determining how to partition blocks for motionestimation, the motion estimator (550) can apply a top-down approach orbottom-up approach, as described below. The motion estimator (550) canuse a search pattern as described below or other search pattern. Whenmultiple reference pictures are used, the multiple reference picturescan be from different temporal directions or the same temporaldirection. The motion estimator (550) produces as side informationmotion data (552) such as MV data, merge mode index values, andreference picture selection data. The motion data (552) is provided tothe header formatter/entropy coder (590) as well as the motioncompensator (555).

The motion compensator (555) applies MVs to the reconstructed referencepicture(s) from the decoded picture buffer (570). The motion compensator(555) produces motion-compensated predictions for the current picture.

In a separate path within the encoder (500), an intra-picture estimator(540) determines how to perform intra-picture prediction for blocks ofsample values of a current picture of the input video signal (505). Thecurrent picture can be entirely or partially coded using intra-picturecoding. Using values of a reconstruction (538) of the current picture,for intra spatial prediction, the intra-picture estimator (540)determines how to spatially predict sample values of a current block ofthe current picture from neighboring, previously reconstructed samplevalues of the current picture.

Or, for intra BC prediction using BV values, the intra-picture estimator(540) estimates displacement of the sample values of the current blockto different candidate reference regions within the current picture.When determining how to partition blocks for BV estimation (and intra BCprediction), the intra-picture estimator (540) can apply a top-downapproach or bottom-up approach, as described below. The intra-pictureestimator (540) can use a search pattern as described below or othersearch pattern. For intra BC prediction, the intra-prediction estimator(540) can constrain the BV selection process using one or moreconstraints described below.

Depending on implementation, the encoder can perform BV estimation forthe current block using input sample values, reconstructed sample valuesbefore in-loop filtering, or reconstructed sample values after in-loopfiltering. In general, by using input sample values or unfiltered,reconstructed sample values for BV estimation, the encoder can avoid asequential-processing bottleneck (which may result from filteringreconstructed sample values of a reference region before BVestimation/intra BC prediction). On the other hand, storing theunfiltered, reconstructed sample values uses additional memory. Also, ifin-loop filtering is applied prior to BV estimation, there may be aregion of influence that overlaps between the filtering process thatwill be applied after the current block region is decoded and the regionbeing used for BV estimation/intra BC prediction. In such a case, the BVestimation/intra BC prediction would be applied before that aspect ofthe filtering operation. In some implementations, the encoder can applysome in-loop filtering operations before BV estimation/intra BCprediction, and perform additional or alternative filtering in a laterprocessing stage.

Or, for an intra-picture dictionary coding mode, pixels of a block areencoded using previous sample values stored in a dictionary or otherlocation, where a pixel is a set of co-located sample values (e.g., anRGB triplet or YUV triplet). For example, the encoder (500) cancalculate hash values of previously reconstructed sample values (e.g.,groupings of 1 pixel, 2 pixels, 4 pixels, 8 pixels, and so on) andcompare those hash values to a hash value of a set of current pixelsbeing encoded. Matches of length one or more can be identified in thepreviously reconstructed sample values based on the hash comparison. Thecurrent pixel(s) (or sample values) can be encoded in various 1-D andpseudo 2-D dictionary modes, using an offset that identifies a locationwithin previous pixels (e.g., in a dictionary) and a length indicating anumber of pixels being predicted from that offset. Typically, noresidual is calculated for a block encoded in intra-picture dictionarycoding mode.

The intra-picture estimator (540) produces as side information intraprediction data (542), such as information indicating whether intraprediction uses spatial prediction, intra BC prediction or a dictionarymode, prediction mode direction (for intra spatial prediction), BVvalues (for intra BC prediction) and offsets and lengths (for dictionarymode). The intra prediction data (542) is provided to the headerformatter/entropy coder (590) as well as the intra-picture predictor(545).

According to the intra prediction data (542), the intra-picturepredictor (545) spatially predicts sample values of a current block ofthe current picture from neighboring, previously reconstructed samplevalues of the current picture. Or, for intra BC prediction, theintra-picture predictor (545) predicts the sample values of the currentblock using previously reconstructed sample values of an intra-pictureprediction reference region, which is indicated by a BV value for thecurrent block. In some cases, the BV value can be a BV predictor(predicted BV value). In other cases, the BV value can be different thanits predicted BV value, in which case a BV differential indicates thedifference between the predicted BV value and BV value. Or, forintra-picture dictionary mode, the intra-picture predictor (545)reconstructs pixels using offsets and lengths.

The intra/inter switch selects whether the prediction (558) for a givenblock will be a motion-compensated prediction or intra-pictureprediction.

For non-dictionary mode, when residual coding is not skipped, thedifference (if any) between a block of the prediction (558) and acorresponding part of the original current picture of the input videosignal (505) provides values of the residual (518). Duringreconstruction of the current picture, when residual values have beenencoded/signaled, reconstructed residual values are combined with theprediction (558) to produce an approximate or exact reconstruction (538)of the original content from the video signal (505). (In lossycompression, some information is lost from the video signal (505).)

In the transformer/scaler/quantizer (530), for non-dictionary modes,when a frequency transform is not skipped, a frequency transformerconverts spatial-domain video information into frequency-domain (i.e.,spectral, transform) data. For block-based video coding, the frequencytransformer applies a discrete cosine transform (“DCT”), an integerapproximation thereof, or another type of forward block transform (e.g.,a discrete sine transform or an integer approximation thereof) to blocksof prediction residual data (or sample value data if the prediction(558) is null), producing blocks of frequency transform coefficients.The transformer/scaler/quantizer (530) can apply a transform withvariable block sizes. In this case, the transformer/scaler/quantizer(530) can determine which block sizes of transforms to use for theresidual values for a current block. The scaler/quantizer scales andquantizes the transform coefficients. For example, the quantizer appliesdead-zone scalar quantization to the frequency-domain data with aquantization step size that varies on a picture-by-picture basis,tile-by-tile basis, slice-by-slice basis, block-by-block basis,frequency-specific basis or other basis. The quantized transformcoefficient data (532) is provided to the header formatter/entropy coder(590). If the frequency transform is skipped, the scaler/quantizer canscale and quantize the blocks of prediction residual data (or samplevalue data if the prediction (558) is null), producing quantized valuesthat are provided to the header formatter/entropy coder (590).

In the scaler/inverse transformer (535), for non-dictionary modes, ascaler/inverse quantizer performs inverse scaling and inversequantization on the quantized transform coefficients. When the transformstage has not been skipped, an inverse frequency transformer performs aninverse frequency transform, producing blocks of reconstructedprediction residual values or sample values. If the transform stage hasbeen skipped, the inverse frequency transform is also skipped. In thiscase, the scaler/inverse quantizer can perform inverse scaling andinverse quantization on blocks of prediction residual data (or samplevalue data), producing reconstructed values. When residual values havebeen encoded/signaled, the encoder (500) combines reconstructed residualvalues with values of the prediction (558) (e.g., motion-compensatedprediction values, intra-picture prediction values) to form thereconstruction (538). When residual values have not beenencoded/signaled, the encoder (500) uses the values of the prediction(558) as the reconstruction (538).

For intra-picture prediction, the values of the reconstruction (538) canbe fed back to the intra-picture estimator (540) and intra-picturepredictor (545). The values of the reconstruction (538) can be used formotion-compensated prediction of subsequent pictures. The values of thereconstruction (538) can be further filtered. A filtering control (560)determines how to perform deblock filtering and SAO filtering on valuesof the reconstruction (538), for a given picture of the video signal(505). The filtering control (560) produces filter control data (562),which is provided to the header formatter/entropy coder (590) andmerger/filter(s) (565).

In the merger/filter(s) (565), the encoder (500) merges content fromdifferent tiles into a reconstructed version of the picture. The encoder(500) selectively performs deblock filtering and SAO filtering accordingto the filter control data (562), so as to adaptively smoothdiscontinuities across boundaries in the pictures. Other filtering (suchas de-ringing filtering or ALF; not shown) can alternatively oradditionally be applied. Tile boundaries can be selectively filtered ornot filtered at all, depending on settings of the encoder (500), and theencoder (500) may provide syntax within the coded bitstream to indicatewhether or not such filtering was applied. The decoded picture buffer(570) buffers the reconstructed current picture for use in subsequentmotion-compensated prediction.

The header formatter/entropy coder (590) formats and/or entropy codesthe general control data (522), quantized transform coefficient data(532), intra prediction data (542), motion data (552) and filter controldata (562). For the motion data (552), the header formatter/entropycoder (590) can select and entropy code merge mode index values, or adefault MV predictor can be used. In some cases, the headerformatter/entropy coder (590) also determines MV differentials for MVvalues (relative to MV predictors for the MV values), then entropy codesthe MV differentials, e.g., using context-adaptive binary arithmeticcoding. For the intra prediction data (542), a BV value can be encodedusing BV prediction. BV prediction can use a default BV predictor (e.g.,from one or more neighboring blocks). When multiple BV predictors arepossible, a BV predictor index can indicate which of the multiple BVpredictors to use for BV prediction. The header formatter/entropy coder(590) can select and entropy code BV predictor index values (for intraBC prediction), or a default BV predictor can be used. In some cases,the header formatter/entropy coder (590) also determines BVdifferentials for BV values (relative to BV predictors for the BVvalues), then entropy codes the BV differentials, e.g., usingcontext-adaptive binary arithmetic coding.

The header formatter/entropy coder (590) provides the encoded data inthe coded video bitstream (595). The format of the coded video bitstream(595) can be a variation or extension of H.265/HEVC format, WindowsMedia Video format, VC-1 format, MPEG-x format (e.g., MPEG-1, MPEG-2, orMPEG-4), H.26x format (e.g., H.261, H.262, H.263, H.264), or anotherformat.

Depending on implementation and the type of compression desired, modulesof an encoder (500) can be added, omitted, split into multiple modules,combined with other modules, and/or replaced with like modules. Inalternative embodiments, encoders with different modules and/or otherconfigurations of modules perform one or more of the describedtechniques. Specific embodiments of encoders typically use a variationor supplemented version of the encoder (500). The relationships shownbetween modules within the encoder (500) indicate general flows ofinformation in the encoder; other relationships are not shown for thesake of simplicity.

VI. EXAMPLE VIDEO DECODERS

FIG. 6 is a block diagram of a generalized decoder (600) in conjunctionwith which some described embodiments may be implemented. The decoder(600) receives encoded data in a coded video bitstream (605) andproduces output including pictures for reconstructed video (695). Theformat of the coded video bitstream (605) can be a variation orextension of HEVC format, Windows Media Video format, VC-1 format,MPEG-x format (e.g., MPEG-1, MPEG-2, or MPEG-4), H.26x format (e.g.,H.261, H.262, H.263, H.264), or another format.

The decoder (600) is block-based and uses a block format that depends onimplementation. Blocks may be further sub-divided at different stages.For example, a picture can be divided into 64×64 blocks, 32×32 blocks or16×16 blocks, which can in turn be divided into smaller blocks of samplevalues. In implementations of decoding for the HEVC standard, a pictureis partitioned into CTUs (CTBs), CUs (CBs), PUs (PBs) and TU (TBs).Blocks (e.g., CUs) can be asymmetrically partitioned into smaller blocks(e.g., PUs) for purposes of intra BC prediction, as shown in FIG. 9 .

The decoder (600) decompresses pictures using intra-picture decodingand/or inter-picture decoding. Many of the components of the decoder(600) are used for both intra-picture decoding and inter-picturedecoding. The exact operations performed by those components can varydepending on the type of information being decompressed.

A buffer receives encoded data in the coded video bitstream (605) andmakes the received encoded data available to the parser/entropy decoder(610). The parser/entropy decoder (610) entropy decodes entropy-codeddata, typically applying the inverse of entropy coding performed in theencoder (500) (e.g., context-adaptive binary arithmetic decoding). As aresult of parsing and entropy decoding, the parser/entropy decoder (610)produces general control data (622), quantized transform coefficientdata (632), intra prediction data (642), motion data (652) and filtercontrol data (662). For the intra prediction data (642), if BV predictorindex values are signaled, the parser/entropy decoder (610) can entropydecode the BV predictor index values, e.g., using context-adaptivebinary arithmetic decoding. In some cases, the parser/entropy decoder(610) also entropy decodes BV differentials for BV values (e.g., usingcontext-adaptive binary arithmetic decoding), then combines the BVdifferentials with corresponding BV predictors to reconstruct the BVvalues. In other cases, the BV differential is omitted from thebitstream, and the BV value is simply the BV predictor (e.g., indicatedwith the BV predictor index value).

The general decoding control (620) receives the general control data(622) and provides control signals (not shown) to other modules (such asthe scaler/inverse transformer (635), intra-picture predictor (645),motion compensator (655) and intra/inter switch) to set and changedecoding parameters during decoding.

If the current picture is predicted using inter-picture prediction, amotion compensator (655) receives the motion data (652), such as MVdata, reference picture selection data and merge mode index values. Themotion compensator (655) applies MVs to the reconstructed referencepicture(s) from the decoded picture buffer (670). The motion compensator(655) produces motion-compensated predictions for inter-coded blocks ofthe current picture. The decoded picture buffer (670) stores one or morepreviously reconstructed pictures for use as reference pictures.

In a separate path within the decoder (600), the intra-frame predictionpredictor (645) receives the intra prediction data (642), such asinformation indicating whether intra prediction uses spatial prediction,intra BC prediction or dictionary mode, and prediction mode direction(for intra spatial prediction), BV values (for intra BC prediction) oroffsets and lengths (for dictionary mode). For intra spatial prediction,using values of a reconstruction (638) of the current picture, accordingto prediction mode data, the intra-picture predictor (645) spatiallypredicts sample values of a current block of the current picture fromneighboring, previously reconstructed sample values of the currentpicture. Or, for intra BC prediction using BV values, the intra-picturepredictor (645) predicts the sample values of the current block usingpreviously reconstructed sample values of an intra-frame predictionregion, which is indicated by a BV value for the current block. Or, forintra-picture dictionary mode, the intra-picture predictor (645)reconstructs pixels using offsets and lengths.

The intra/inter switch selects values of a motion-compensated predictionor intra-picture prediction for use as the prediction (658) for a givenblock. For example, when HEVC syntax is followed, the intra/inter switchcan be controlled based on a syntax element encoded for a CU of apicture that can contain intra-predicted CUs and inter-predicted CUs.When residual values have been encoded/signaled, the decoder (600)combines the prediction (658) with reconstructed residual values toproduce the reconstruction (638) of the content from the video signal.When residual values have not been encoded/signaled, the decoder (600)uses the values of the prediction (658) as the reconstruction (638).

To reconstruct the residual when residual values have beenencoded/signaled, the scaler/inverse transformer (635) receives andprocesses the quantized transform coefficient data (632). In thescaler/inverse transformer (635), a scaler/inverse quantizer performsinverse scaling and inverse quantization on the quantized transformcoefficients. An inverse frequency transformer performs an inversefrequency transform, producing blocks of reconstructed predictionresidual values or sample values. For example, the inverse frequencytransformer applies an inverse block transform to frequency transformcoefficients, producing sample value data or prediction residual data.The inverse frequency transform can be an inverse DCT, an integerapproximation thereof, or another type of inverse frequency transform(e.g., an inverse discrete sine transform or an integer approximationthereof). If the frequency transform was skipped during encoding, theinverse frequency transform is also skipped. In this case, thescaler/inverse quantizer can perform inverse scaling and inversequantization on blocks of prediction residual data (or sample valuedata), producing reconstructed values.

For intra-picture prediction, the values of the reconstruction (638) canbe fed back to the intra-picture predictor (645). For inter-pictureprediction, the values of the reconstruction (638) can be furtherfiltered. In the merger/filter(s) (665), the decoder (600) mergescontent from different tiles into a reconstructed version of thepicture. The decoder (600) selectively performs deblock filtering andSAO filtering according to the filter control data (662) and rules forfilter adaptation, so as to adaptively smooth discontinuities acrossboundaries in the frames. Other filtering (such as de-ringing filteringor ALF; not shown) can alternatively or additionally be applied. Tileboundaries can be selectively filtered or not filtered at all, dependingon settings of the decoder (600) or a syntax indication within theencoded bitstream data. The decoded picture buffer (670) buffers thereconstructed current picture for use in subsequent motion-compensatedprediction.

The decoder (600) can also include a post-processing filter. Thepost-processing filter (608) can include deblock filtering, de-ringingfiltering, adaptive Wiener filtering, film-grain reproduction filtering,SAO filtering or another kind of filtering. Whereas “in-loop” filteringis performed on reconstructed sample values of frames in a motioncompensation loop, and hence affects sample values of reference frames,the post-processing filter (608) is applied to reconstructed samplevalues outside of the motion compensation loop, before output fordisplay.

Depending on implementation and the type of decompression desired,modules of the decoder (600) can be added, omitted, split into multiplemodules, combined with other modules, and/or replaced with like modules.In alternative embodiments, decoders with different modules and/or otherconfigurations of modules perform one or more of the describedtechniques. Specific embodiments of decoders typically use a variationor supplemented version of the decoder (600). The relationships shownbetween modules within the decoder (600) indicate general flows ofinformation in the decoder; other relationships are not shown for thesake of simplicity.

VII. INTRA BLOCK COPY PREDICTION WITH ASYMMETRIC PARTITIONS

This section presents examples of asymmetric partitions forintra-BC-predicted blocks. Using asymmetric partitions can allow anencoder to adapt the partitions to strongly defined but irregular imagepatterns within blocks, which are common in text, Web pages, and otherparts of screen capture video and other artificially-created video.

A. Intra BC Prediction Mode and BV Values—Introduction.

For intra BC prediction, the sample values of a current block of apicture are predicted using sample values in the same picture. A BVvalue indicates a displacement from the current block to a region of thepicture (the “reference region”) that includes the sample values usedfor prediction. The reference region provides predicted values for thecurrent block. The sample values used for prediction are previouslyreconstructed sample values, which are thus available at the encoderduring encoding and at the decoder during decoding. The BV value issignaled in the bitstream, and a decoder can use the BV value todetermine the reference region of the picture to use for prediction,which is also reconstructed at the decoder. Intra BC prediction is aform of intra-picture prediction—intra BC prediction for a block of apicture does not use any sample values other than sample values in thesame picture.

FIG. 7 illustrates intra BC prediction for a current block (730) of acurrent picture (710). The current block can be a coding block (“CB”) ofa coding unit (“CU”), prediction block (“PB”) of a prediction unit(“PU”), transform block (“TB”) of a transform unit (“TU”) or otherblock. The size of the current block can be 64×64, 32×32, 16×16, 8×8 orsome other size. More generally, the size of the current block is m×n,where each of m and n is a whole number, and where m and n can be equalto each other or can have different values. Thus, the current block canbe square or rectangular. Alternatively, the current block can have someother shape.

The BV (740) indicates a displacement (or offset) from the current block(730) to a reference region (750) of the picture that includes thesample values used for prediction. The reference region (750) indicatedby the BV (740) is sometimes termed the “matching block” for the currentblock (730). The matching block can be identical to the current block(730), or it can be an approximation of the current block (730). Supposethe top-left position of a current block is at position (x₀, y₀) in thecurrent picture, and suppose the top-left position of the referenceregion is at position (x₁, y₁) in the current picture. The BV indicatesthe displacement (x₁−x₀, y₁−y₀). For example, if the top-left positionof the current block is at position (256, 128), and the top-leftposition of the reference region is at position (126, 104), the BV valueis (−130, −24). In this example, a negative horizontal displacementindicates a position to the left of the current block, and a negativevertical displacement indicates a position above the current block.

Intra BC prediction can improve coding efficiency by exploitingredundancy (such as repeated patterns inside a picture) using BCoperations. Finding a matching block for a current block can becomputationally complex and time consuming, however, considering thenumber of candidate blocks that the encoder may evaluate. FIG. 8 showssome of the candidate blocks for a current block (830) of a currentpicture (810) in block matching operations. Four BVs (841, 842, 843,844) indicate displacements for four candidate blocks. The candidateblocks can be anywhere within the reconstructed content of the currentpicture (810). (Blocks are generally coded from left-to-right, then fromtop-to-bottom.) A candidate block can overlap with other candidateblocks, as shown for the candidate blocks indicated by the BVs (843,844).

In some example implementations, the intra-predicted region (850) isconstrained to be within the same slice and tile as the current block(830). Such intra BC prediction does not use sample values in otherslices or tiles. The location of the intra-predicted region (850) may besubject to one or more other constraints (e.g., for search range,regarding use of reconstructed sample values of inter-coded blocks).Alternatively, the location of the intra-predicted region (850) is notconstrained (that is, full search range) within the reconstructedcontent of the current picture (810).

A block with prediction mode of intra BC prediction can be a CB, PB orother block. When the block is a CB, the BV for the block can besignaled at CU level (and other CBs in the CU use the same BV or ascaled version thereof). Or, when the block is a PB, the BV for theblock can be signaled at PU level (and other PBs in the PU use the sameBV or a scaled version thereof). More generally, the BV for an intra-BCprediction block is signaled at an appropriate syntax level for theblock.

The block copying operations of prediction according to the intra BCprediction mode can be performed at the level of CB (when a BV issignaled per CB) or PB (when a BV is signaled per PB). For example,suppose a 16×16 CB has a single 16×16 PB. The BV (for the PB) is appliedto block copy a 16×16 region. When the intra-prediction region isconstrained to not overlap the 16×16 block being predicted, the BV has amagnitude (absolute value) of at least 16 horizontally or vertically.

Alternatively, the block copying operations can be performed at thelevel of TBs within a PB or CB, even when the BV is signaled for the PBor CB. In this way, a BV, as applied for a TB, can reference positionsof other TBs in the same PB or CB. For example, suppose a 16×16 CB has asingle 16×16 PB but is split into sixteen 4×4 TBs for purposes ofresidual coding/decoding. The BV (for the PB) is applied to block copy a4×4 region for the first TB in raster scan order, then the same BV isapplied to block copy a 4×4 region for the second TB in raster scanorder, and so on. The 4×4 region used in the BC operations for a TB caninclude positions in previously reconstructed TBs in the same CB, aftercombining residual values with predicted values for those previouslyreconstructed TBs. (A BV still does not reference positions in the sameTB that is being predicted). Applying BC operations at the TB levelfacilitates use of BVs with relatively small magnitudes.

TB-level overlapping creates serial dependencies in reconstruction forthe TUs within a PU, implicitly. This reduces opportunities for parallelcomputing and potentially decreases throughput at the hardware level.PU-level overlapping can eliminate such serial dependencies among TBs.In this case, block copying operations are performed at the level of PBsthat can overlap. In this way, a BV, as applied for a PB, can referencepositions of other PBs in the same CB. Suppose a 16×16 CB is split intotwo PBs for purposes of intra BC prediction (e.g., two 16×8 PBs, or two8×16 PBs, or a 4×16 PB and 12×16 PB, etc.). The BV for one PB is appliedto block copy a region for that PB, then the BV for the other PB isapplied to block copy a region for the other PB. The region used in theBC operations for the second PB can include positions in the previouslyreconstructed first PB in the same CB, after combining residual valueswith predicted values for the first PB. (A BV still does not referencepositions in the same PB that is being predicted). Applying BCoperations at the PB level facilitates use of BVs with relatively smallmagnitudes (compared to applying BC operations at the CB level). Also,when BC operations are applied at the PB level, TU-level parallelprocessing is still allowed for the TBs within a PB.

Intra BC prediction operations for chroma blocks of a CU generallycorrespond to intra BC prediction operations for the luma block of theCU. Normally, the segmentation of chroma PBs and chroma TBs correspondsdirectly to the segmentation of the luma PBs and luma TBs in the CU.When the format of video is YUV 4:4:4, the sizes of chroma PBs and TBsmatch the sizes of corresponding luma PBs and TBs. When the format ofvideo is YUV 4:2:0, chroma PBs and TBs are half the width and half theheight of corresponding luma PBs and TBs. If a luma TB has minimumtransform size, however, a single chroma TB having that minimumtransform size is used. When the format of video is YUV 4:2:2, chromaPBs and TBs are half the width of corresponding luma PBs and TBs.

In some implementations, for an intra BC predicted CU, intra BCprediction for a chroma block in a PU uses the same BV value as intra BCprediction for the luma block in the PU, possibly after scaling androunding when the chroma data has reduced resolution relative to theluma data (e.g. when the format is YUV 4:2:0 format, the BV value isdivided by two for horizontal and vertical components; or, when theformat is YUV 4:2:2 format, the BV value is divided by two for thehorizontal component). Alternatively, different BV values can besignaled for the luma block and chroma blocks of a PU.

In some implementations, an encoder considers luma sample values whenidentifying a BV value or MV value during BV estimation or MVestimation. For example, the encoder attempts to match luma samplevalues for a current block (e.g., PB of a PU) to reconstructed lumasample values. The resulting BV value or MV value is also applied tochroma sample values of corresponding chroma blocks, however.Alternatively, an encoder considers luma sample values and correspondingchroma sample values when identifying a BV value or MV value during BVestimation or MV estimation.

In some implementations, if the prediction mode of the luma block of aPU is intra BC prediction, the prediction mode for the chroma blocks ofthe PU is also intra BC predicted. For example, the prediction mode issignaled for the PU. Alternatively, the prediction mode can be intra BCprediction for the luma block or chroma blocks of the PU, but not both.

B. Asymmetric Partitions.

FIG. 9 shows examples (900) of partitions of a block for intra BCprediction in some example implementations. A 2N×2N block is encodedusing intra BC prediction. For example, the 2N×2N block is a 64×64block, 32×32 block, 16×16 block or 8×8 block. The 2N×2N block can beintra-BC-predicted without partitioning. Or, the 2N×2N block can bepartitioned in various ways, as shown in FIG. 9 .

The 2N×2N block can be partitioned horizontally into two partitions. Thetwo partitions can have the same dimensions—two 2N×N blocks—forsymmetric partitioning. Or, the two partitions can be asymmetric. Forexample, the upper partition is a 2N×N/2 block, and the lower partitionis a 2N×3N/2 block. Or, as another example, the upper partition is a2N×3N/2 block, and the lower partition is a 2N×N/2 block. Thus, a 64×64block can be partitioned into two 64×32 blocks, a 64×16 block and 64×48block, or a 64×48 block and 64×16 block. A 32×32, 16×16 or 8×8 block cansimilarly be horizontally partitioned.

The 2N×2N block can instead be partitioned vertically into twopartitions. The two partitions can have the same dimensions—two N×2Nblocks—for symmetric partitioning. Or, the two partitions can beasymmetric. For example, the left partition is an N/2×2N block, and theright partition is a 3N/2×2N block. Or, as another example, the leftpartition is a 3N/2×2N block, and the right partition is an N/2×2Nblock. Thus, a 64×64 block can be partitioned into two 32×64 blocks, a16×64 block and 48×64 block, or a 48×64 block and 16×64 block. A 32×32,16×16 or 8×8 block can similarly be vertically partitioned.

Or, the 2N×2N block can be partitioned into four N×N partitions, whichmay be further sub-divided. For example, as shown in FIG. 9 , a givenN×N partition can be further partitioned into two N×N/2 blocks, twoN/2×N blocks or four N/2×N/2 blocks. Thus, a 64×64 block can bepartitioned into four 32×32 blocks, each of which may be furtherpartitioned into two 32×16 blocks, two 16×32 blocks or four 16×16blocks. A 32×32, 16×16 or 8×8 block can similarly be partitioned byquadtree splitting into four partitions.

As the term is used herein, an “N×1V” partition can also be considered a2N×2N partition, in most cases. In H.265/HEVC implementations, the termN×N is typically used to describe a PU or PB, but not a CU or CB. Asused herein, the term “N×N partition” or “N×N block” indicates apartition of a 2N×2N current block (e.g., as part of top-down evaluationof partition modes or bottom-up evaluation of partition modes). In thiscontext, the N×N partition or N×N block can itself be considered a 2N×2Nblock, and may be treated as such in further partitioning, unless theN×N partition or N×N block has the minimum size and is not furtherpartitioned. Notation for partitions within an N×N partition or N×Nblock can similarly be adjusted.

Limiting asymmetric partition sizes to multiples of N/2 can reduce thecomplexity of evaluating which partition modes to use during encoding.Alternatively, an encoder can consider other partition sizes (e.g.,multiples of N/4 or partition sizes m×n, more generally). Consideringother partition sizes may slightly improve coding gains, but alsoincreases the complexity of the search process during encoding, and mayincrease signaling overhead.

C. Example Techniques for Encoding or Decoding that Includes Intra BCPrediction with Asymmetric Partitions.

FIG. 10 shows a generalized technique (1000) for encoding that includesintra BC prediction with asymmetric partitions. An image encoder orvideo encoder such as described with reference to FIG. 3 or FIGS. 5 a-5b can perform the technique (1000). FIG. 11 shows a generalizedtechnique (1100) for decoding that includes intra BC prediction withasymmetric partitions. An image decoder or video decoder such asdescribed with reference to FIG. 4 or FIG. 6 can perform the technique(1100).

With reference to FIG. 10 , an encoder encodes (1010) an image or videoto produce encoded data. As part of the encoding, the encoder performsintra BC prediction for a current block that is asymmetricallypartitioned for the intra BC prediction. In particular, the currentblock can be split into two partitions that have different dimensions.The encoder outputs (1020) the encoded data as part of a bitstream.

With reference to FIG. 11 , a decoder receives (1110) encoded data aspart of a bitstream. The decoder decodes (1120) the encoded data toreconstruct an image or video. As part of the decoding, the decoderperforms intra BC prediction for a current block that is asymmetricallypartitioned for the intra BC prediction. In particular, the currentblock can be split into two partitions that have different dimensions.

For example, a current 2N×2N block is horizontally partitioned into (1)a 2N×N/2 block and 2N×3N/2 block or (2) a 2N×3N/2 block and 2N×N/2block. Or, a current 2N×2N block is vertically partitioned into (1) anN/2×2N block and 3N/2×2N block or (2) a 3N/2×2N block and N/2×2N block.Alternatively, the current block is asymmetrically partitioned in someother way.

Another block can be symmetrically partitioned for the intra BCprediction. For example, the other block is a 2N×2N block that ispartitioned into (1) two 2N×N blocks, (2) two N×2N blocks, or (3) fourN×N blocks, each of which can be further partitioned into two N×N/2blocks, two N/2×N blocks or four N/2×N/2 blocks. More generally, forsymmetric partitioning, the other block can be split into partitionsthat have identical dimensions.

VIII. ENCODER-SIDE APPROACHES TO PARTITIONING

This section presents various encoder-side approaches to identifyingpartitions during motion estimation or block vector estimation (forintra BC prediction).

Examples of intra BC prediction and BV estimation are presented, forexample, in sections V, VII.A, IX and X. BV estimation can becomputationally complex, considering the large number of possiblecandidate blocks for a current block. The computational complexity of BVestimation is especially problematic when the search range for BV valuesencompasses all of the previously reconstructed areas of a picture, dueto the large number of candidate blocks against which a current block iscompared. For candidate blocks of an entire frame, the number ofoperations is even higher. When the encoder evaluates asymmetricpartitions, the complexity of BV estimation is further increased.

The computational complexity of motion estimation can also be very high,as explained in the following section.

A. Motion Estimation and MV Values—Introduction.

For motion estimation, the sample values of a current block of a currentpicture are predicted using sample values in another picture, which iscalled the reference picture. A motion vector (“MV”) value indicates adisplacement from the position of the current block in the referencepicture to a region of the reference picture (the “reference region”)that includes the sample values used for prediction. The referenceregion provides predicted values for the current block. The samplevalues used for prediction are previously reconstructed sample values,which are thus available at the encoder during encoding and at thedecoder during decoding. The MV value is signaled in the bitstream, anda decoder can use the MV value to determine the reference region of thereference picture to use for prediction, which is also reconstructed atthe decoder. When multiple reference pictures are available, thebitstream can also include an indication of which of the referencepicture to use to find the reference region.

FIG. 12 illustrates motion estimation for a current block (1230) of acurrent picture (1210). The current block can be a coding block (“CB”)of a coding unit (“CU”), prediction block (“PB”) of a prediction unit(“PU”), transform block (“TB”) of a transform unit (“TU”) or otherblock. The size of the current block can be 64×64, 32×32, 16×16, 8×8 orsome other size. More generally, the size of the current block is m×n,where each of m and n is a whole number, and where m and n can be equalto each other or can have different values. Thus, the current block canbe square or rectangular. Alternatively, the current block can have someother shape.

The MV value (1240) indicates a displacement (or offset) from theposition of the current block (1230) to a reference region (sometimescalled a reference block) in a reference picture (1250), which includesthe sample values used for prediction. The reference region indicated bythe MV value (1240) is sometimes termed the “matching block” for thecurrent block (1230). The matching block can be identical to the currentblock (1230), or it can be an approximation of the current block (1230).Suppose the top-left position of the current (1230) block is at position(x₀, y₀) in the current picture (1210), and suppose the top-leftposition of the reference region is at position (x₁, y₁) in thereference picture (1250). The MV value (1240) indicates the displacement(x₁−x₀, y₁−y₀). For example, if the top-left position of the currentblock is at position (256, 128), and the top-left position of thereference region is at position (126, 104), the MV value is (−130, −24).In this example, a negative horizontal displacement indicates a positionto the left of the current block, and a negative vertical displacementindicates a position above the current block.

The reference region for the current block (1230) is selected from amongmultiple candidate blocks during motion estimation. FIG. 12 also showssome of the candidate blocks for the current block (1230) of the currentpicture (1210) in motion estimation. Four MV values (1241, 1242, 1243,1244) indicate displacements for four candidate blocks. In general, thecandidate blocks can be anywhere within the reference picture (1250). Acandidate block can overlap with other candidate blocks, as shown forthe candidate blocks indicated by the MV values (1243, 1244). Thecomputational complexity of motion estimation is especially problematicwhen the search range for MV values encompasses all of a referencepicture, due to the large number of candidate blocks against which acurrent block is compared. This computational complexity is compoundedwhen motion estimation is performed for multiple reference pictures.

B. Precision of BV Values and MV Values.

In many of the examples described herein, BV values have integer-sampleprecision. Such BV values indicate integer-sample offsets. Intra BCprediction is often used when encoding artificially-created video (suchas screen content video), for which fractional-sample displacements arerare, so integer-sample precision is sufficient for BV values.Alternatively, BV values can indicate fractional-sample offsets. Forexample, a BV value with ½-sample precision can indicate a horizontaland/or vertical displacement with a ½-sample offset (such as 1.5samples, 2.5 samples, and so on). Or, a BV value with ¼-sample precisioncan indicate a horizontal and/or vertical displacement with a ¼, ½ or¾-sample offset. Or, a BV value with ⅛-sample precision can indicate ahorizontal and/or vertical displacement with a ⅛, ¼, ⅜, ½, ⅝, ¾, or⅞-sample offset. Or, a BV value can have some other precision.

On the other hand, MV values typically have fractional-sample precision,since fractional-sample displacements are common when encoding naturalvideo. For example, MV values can have ½-sample precision, ¼-sampleprecision, ⅛-sample precision or some other precision. Alternatively, MVvalues have integer-sample precision.

C. Approaches to Identifying Partitions.

In general, an encoder can use a top-down approach or bottom-up approachwhen identifying partitions during motion estimation or BV estimation(for intra BC prediction). A bottom-up approach initially evaluatesoptions for smallest-size partitions, then uses results from thatinitial evaluation when evaluating options for successively larger-sizepartitions. A bottom-up approach can be computationally expensive. Onthe other hand, a top-down approach initially evaluates options forlarger-size partitions, then uses results from that initial evaluationwhen evaluating options for successively smaller-size partitions. Atop-down approach is less likely to identify an optimal way to partitionblocks, but is usually computationally simpler than a bottom-upapproach.

For additional details about partitioning approaches, see, e.g.: (1)Sullivan et al., “Efficient Quadtree Coding of Images and Video”, inProc. IEEE Int. Conf. on Acoust., Speech, and Signal Proc. (ICASSP),Toronto, Canada, Vol. 4, pp. 2661-2664, May 1991, which describes how toidentify an optimal tree in the rate-distortion sense fortree-structured coding (at least under some circumstances); (2) Sullivanet al., “Rate-Distortion Optimized Motion Compensation for VideoCompression using Fixed or Variable Size Blocks”, in Proc. IEEE GlobalTelecom. Conf. (GLOBECOM), Phoenix, Ariz., pp. 85-90, December 1991,which describes how to apply rate-distortion optimization to motioncompensation (with or without trees); and (3) Sullivan et al.,“Efficient Quadtree Coding of Images and Video”, IEEE Trans. on ImageProc., Vol. IP-3, No. 3, pp. 327-331, May 1994, which further describeshow to identify an optimal tree in the rate-distortion sense fortree-structured coding.

In some implementations, applying one of the top-down approaches orbottom-up approaches described below, an encoder identifies partitionsfor a PU. The encoder can set the partitions for a PU based on analysisof luma PBs of the PU. Or, the encoder can set the partitions for a PUbased on analysis of both luma PBs and chroma PBs of the PU. In anycase, the partitions set on a PU basis are then applied to PBs of thePU. Alternatively, an encoder identifies partitions for another type ofunit or block.

1. Top-down Approaches to Identifying Partitions.

An encoder can use a top-down approach when identifying partitions of acurrent block during BV estimation (for intra BC prediction) or motionestimation. The current block can be a PB of a PU, or other type ofblock. FIG. 13 is a flowchart and accompanying diagram illustrating atop-down approach to partitioning an intra-BC-predicted block. An imageencoder or video encoder such as described with reference to FIG. 3 orFIGS. 5 a-5 b can use the approach (1300). In the example shown in FIG.13 , the encoder identifies at least some partitions of anintra-BC-predicted 2N×2N block with asymmetric partitioning.

The encoder checks (1310) modes with a 2N-dimension. For example, theencoder checks a mode for a single 2N×2N block, a mode for two 2N×Nblocks, a mode for two N×2N blocks, modes for one N/2×2N block and one3N/2×2N block (two options shown in FIG. 13 , with narrow block at leftor right) and modes for one 2N×N/2 block and one 2N×3N/2 block (twooptions shown in FIG. 13 , with shorter block at top or bottom). For a16×16 CU, for example, the encoder checks all of the PUs with size 16×Pand all of the PUs with size P×16, where P can be 4, 8, 12 and 16, inthe allowed combinations.

For a given mode for the current 2N×2N block, the encoder determines BVvalue(s) for block(s) of the current 2N×2N block according to the mode.For a block of the current 2N×2N block, the encoder can select astarting BV value, for example, based on (1) the BV value(s) used byneighboring block(s) in the current picture, (2) the BV value used by acollocated block of a previous picture, or (3) an MV value identifiedfor the block of the current 2N×2N block in earlier motion estimation.The encoder then finds a suitable BV value for the block of the current2N×2N block.

The encoder selects (1320) the best mode with a 2N-dimension. Theselection criterion can be distortion cost, bit rate cost or somecombination of distortion cost and bit rate cost, or the selectioncriterion can use some other metric (e.g., using a variance threshold oredge detector). For example, in FIG. 13 , the encoder selects the modewith a left N/2×2N block and right 3N/2×2N block.

After splitting of the 2N×2N block as a quadtree, the encoder alsochecks (1330) modes per N×N block of the 2N×2N block. For example, for agiven N×N block, the encoder checks a mode for a single N×N block, amode for two N×N/2 blocks, a mode for two N/2×N blocks, and a mode forfour N/2×N/2 blocks. The encoder can check each N×N block separately.For an 8×8 CU, for example, the encoder checks an 8×8 PU, two 8×4 PUs,two 4×8 PUs and four 4×4 PUs.

For a given mode for a given N×N block, the encoder determines BVvalue(s) for block(s) of the given N×N block according to the mode. Theencoder can select starting BV values, for example, based on the BVvalues that were identified for the current 2N×2N block. The encoderthen finds suitable BV value(s) for the block(s) of the given N×N blockaccording to the mode.

The encoder selects (1340) the best combination of modes for therespective N×N blocks. The selection criterion can be distortion cost,bit rate cost or some combination of distortion cost and bit rate cost,or the selection criterion can use some other metric (e.g., using avariance threshold or edge detector). As shown in FIG. 13 , differentN×N blocks can have the same mode or different modes.

For the 2N×2N block, the encoder then selects (1350) between the bestmode with a 2N-dimension and the combination of best modes for therespective N×N blocks. The selection criterion can be distortion cost,bit rate cost or some combination of distortion cost and bit rate cost,or the selection criterion can use some other metric (e.g., using avariance threshold or edge detector).

2. Bottom-up Approaches to Identifying Partitions

Instead of using a top-down approach, an encoder can use a bottom-upapproach when identifying partitions of a current block during BVestimation (for intra BC prediction) or motion estimation. FIG. 14 is aflowchart illustrating a generalized technique for using a bottom-upapproach to partitioning. An image encoder or video encoder such asdescribed with reference to FIG. 3 or FIGS. 5 a-5 b can perform thetechnique (1400).

The encoder encodes (1410) an image or video to produce encoded data. Aspart of the encoding (e.g., as part of motion estimation or as part ofblock vector estimation for intra BC prediction), the encoder computes aprediction for a current block of a current picture. The current blockcan be a PB of a PU, or other type of block. When computing theprediction, the encoder uses a bottom-up approach to identify partitionsof the current block. The encoder can asymmetrically partition thecurrent block, such that the partitions of the current block are twopartitions that have different dimensions, or symmetrically partitionthe current block, such that the partitions of the current block haveidentical dimensions.

In general, for the bottom-up approach, for a current block, the encoderchecks at least some partition modes for smaller blocks within thecurrent block. The encoder caches results such as BV values or MV valuesfor the smaller blocks. The encoder then checks at least some partitionmodes for the current block, using the cached results (from the smallerblocks) to reduce the computational complexity of checking the partitionmodes for the current block. For example, the encoder uses the cachedresults from the smaller blocks to identify starting BV values (duringBV estimation) or MV values (during motion estimation) for the partitionmodes for the current block. In many cases, the starting BV values (orMV values) will be used for the current block, which significantlyshortens the search process. FIGS. 15 and 16 show example bottom-upapproaches to identifying partitions for a current block. Alternatively,the encoder uses another bottom-up approach.

The encoder then outputs (1420) the encoded data as part of a bitstream.

FIG. 15 is a flowchart and accompanying diagram illustrating a bottom-upapproach (1500) to partitioning an intra-BC-predicted block. An imageencoder or video encoder such as described with reference to FIG. 3 orFIGS. 5 a-5 b can use the approach (1500).

For a 2N×2N block that is split as a quadtree, the encoder checks (1510)modes per N×N block of the 2N×2N block. For example, for a given N×Nblock, the encoder checks a mode for a single N×N block, a mode for twoN×N/2 blocks, a mode for two N/2×N blocks, and a mode for four N/2×N/2blocks. The encoder can check each N×N block separately. For an 8×8 CU,for example, the encoder checks modes for an 8×8 PU, two 8×4 PUs, two4×8 PUs and four 4×4 PUs.

For a given mode of a given N×N block, the encoder determines BVvalue(s) for block(s) of the given N×N block according to the mode. Fora block of the given N×N block, the encoder can select a starting BVvalue, for example, based on (1) the BV value(s) used by neighboringblock(s) in the current picture, (2) the BV value used by a collocatedblock of a previous picture, or (3) an MV value identified for the blockof the given N×N block in earlier motion estimation. The encoder thenfinds a suitable BV value for the block of the given N×N block.

The encoder selects (1520) the best combination of modes for therespective N×N blocks. The selection criterion can be distortion cost,bit rate cost or some combination of distortion cost and bit rate cost,or the selection criterion can use some other metric (e.g., using avariance threshold or edge detector). As shown in FIG. 15 , differentN×N blocks can have the same mode or different modes.

The encoder caches (1530) vector values, partition mode informationand/or other results of the checking (1510) modes per N×N block of the2N×2N block. For example, the encoder caches BV values during BVestimation, as well as partition mode selections for the N×N blocks ofthe 2N×2N block.

The encoder checks (1540) modes with a 2N-dimension, using the cachedinformation to reduce computational complexity by shortening the searchprocess in many cases. The encoder can use the cached results toidentify starting BV values (during BV estimation) for the 2N×2N block.In many cases, the starting BV values will be used for the 2N-dimensionpartition modes of the 2N×2N block, which significantly shortens thesearch process.

For example, the encoder checks a mode for a single 2N×2N block, a modefor two 2N×N blocks, a mode for two N×2N blocks, modes for one N/2×2Nblock and one 3N/2×2N block (two options shown in FIG. 15 , with narrowblock at left or right), and modes for one 2N×N/2 block and one 2N×3N/2block (two options shown in FIG. 15 , with shorter block at top orbottom). For a 16×16 CU, for example, the encoder reuses informationcached after checking partition modes for the four 8×8 CUs of the 16×16CU. In many cases, a partition mode for the 16×16 CU ends up using acached BV value from an 8×8 CU, which significantly shortens the searchprocess.

The encoder selects (1550) the best mode with a 2N-dimension. Theselection criterion can be distortion cost, bit rate cost or somecombination of distortion cost and bit rate cost, or the selectioncriterion can use some other metric (e.g., using a variance threshold oredge detector). For example, in FIG. 15 the encoder selects the modewith a left N/2×2N block and right 3N/2×2N block.

For the 2N×2N block, the encoder then selects (1560) between the bestmode with a 2N-dimension and the combination of modes for the respectiveN×N blocks. The selection criterion can be distortion cost, bit ratecost or some combination of distortion cost and bit rate cost, or theselection criterion can use some other metric (e.g., using a variancethreshold or edge detector).

The technique (1500) described with reference to FIG. 15 can also beused for bottom-up partitioning of inter-coded blocks. In this case, MVvalues and other results from N×N blocks are cached. An encoder can usethe cached results to identify starting MV values (during motionestimation) for the 2N×2N block. In many cases, the starting MV valueswill be used for the 2N-dimension partition modes of the 2N×2N block,which significantly shortens the search process.

FIG. 16 is a flowchart and accompanying diagram illustrating even fasterbottom-up approaches (1600) to partitioning an intra-BC-predicted block.An image encoder or video encoder such as described with reference toFIG. 3 or FIGS. 5 a-5 b can use one of the approaches (1600). Theapproaches (1600) in FIG. 16 are similar to the approach (1500) in FIG.15 , but have been modified in several places to further shorten thesearch process.

For a 2N×2N block that is split as a quadtree, the encoder checks (1610)a subset of modes per N×N block of the 2N×2N block. That is, the encoderchecks some but not all of the modes per N×N block. For example, for agiven N×N block, the encoder checks only a mode with two N×N/2 blocks.Or, as another example, the encoder checks only a mode with two N/2×Nblocks. The encoder can check each N×N block separately. By checkingfewer modes, however, the search process is shortened. Alternatively,the encoder checks other and/or additional modes per N×N block.

If multiple modes were checked (1610), the encoder selects (1620) thebest combination of modes for the respective N×N blocks. The selectioncriterion can be distortion cost, bit rate cost or some combination ofdistortion cost and bit rate cost, or the selection criterion can usesome other metric (e.g., using a variance threshold or edge detector).If only a single mode was checked (1610), the encoder simply uses thatmode per N×N block.

The encoder caches (1630) vector values, partition mode informationand/or other results of the checking (1610) modes per N×N block of the2N×2N block. For example, the encoder caches BV values during BVestimation, as well as partition mode selections for the N×N blocks ofthe 2N×2N block.

The encoder checks (1640) a subset of modes with a 2N-dimension, usingthe cached information to reduce computational complexity. For example,if the encoder checked (1610) only N×N/2 blocks of the N×N blocks, theencoder checks a mode for a single 2N×2N block, a mode for two 2N×Nblocks, and modes for one 2N×N/2 block and one 2N×3N/2 block (twooptions shown in FIG. 16 , with shorter block at top or bottom). Or, ifthe encoder checked (1610) only N/2×N blocks of the N×N blocks, theencoder checks a mode for a single 2N×2N block, a mode for two N×2Nblocks, and modes for one N/2×2N block and one 3N/2×2N block (twooptions shown in FIG. 16 , with narrow block at left or right).

Or, as another example (not illustrated in FIG. 16 ), the encoder checks(1610) only the N×N partition per N×N block of the 2N×2N block. If the2N×2N block is an intra-coded block, the encoder checks (1640) a modefor a single 2N×2N block, a mode for two N×2N blocks, and modes for oneN/2×2N block and one 3N/2×2N block (with narrow block at left or right).If the 2N×2N block is an inter-coded block, the encoder checks (1640) amode for a single 2N×2N block, a mode for two 2N×N blocks, and modes forone 2N×N/2 block and one 2N×3N/2 block (with shorter block at top orbottom).

The encoder selects (1650) the best mode with a 2N-dimension. Theselection criterion can be distortion cost, bit rate cost or somecombination of distortion cost and bit rate cost, or the selectioncriterion can use some other metric (e.g., using a variance threshold oredge detector). For the 2N×2N block, the encoder then selects (1660)between the best mode with a 2N-dimension and the combination of modesfor the respective N×N blocks. The selection criterion can be distortioncost, bit rate cost or some combination of distortion cost and bit ratecost, or the selection criterion can use some other metric (e.g., usinga variance threshold or edge detector).

The technique (1600) described with reference to FIG. 16 can also beused for bottom-up partitioning of inter-coded blocks. In this case, MVvalues and other results from N×N blocks are cached. An encoder can usethe cached results to identify starting MV values (during motionestimation) for the 2N×2N block. In many cases, the starting MV valueswill be used for the 2N-dimension partition modes of the 2N×2N block,which significantly shortens the search process.

In the approaches (1500, 1600) shown in FIGS. 15 and 16 , the encodercan limit the number of cached BV values or MV values. For example, theencoder stores only the BV value(s) or MV value(s) for the bestpartition mode per N×N block of a 2N×2N block. Alternatively, theencoder stores other BV values or MV values as well. An encoderconstraint can control how many BV values or MV values the encoderstores.

IX. SEARCH PATTERNS

This section presents various search patterns that can be used in motionestimation or intra BC prediction. In particular, the search patternsexploit common types of motion in screen capture video or otherartificially-created video. In such video, the motion for a block isoften pure horizontal motion or pure vertical motion (e.g., fromscrolling of Web page content or application content in a computerdesktop environment, or from scrolling of a ticker graphic in mixedcontent video). In this sense, the search patterns are adapted forscreen capture video or other artificially-created video, but they canalso be used when encoding natural video.

FIG. 17 is a flowchart illustrating a generalized technique (1700) forsearching for a BV value or MV value for a block using iterativeevaluation of a location in small neighborhood(s) and iterativeconfirmation of the location in larger neighborhood(s). An image encoderor video encoder such as described with reference to FIG. 3 or FIGS. 5a-5 b can perform the technique (1700).

The encoder encodes an image or video to produce encoded data, andoutputs the encoded data as part of a bitstream. During the encoding,the encoder computes a prediction for a current block of a currentpicture (e.g., using BV estimation or motion estimation). When computingthe prediction, the encoder identifies (1710) a current best locationfor the prediction through iterative evaluation in a small neighborhoodaround the current best location for the prediction. For example, thesmall neighborhood includes locations that are immediately adjacenthorizontally or vertically to the current best location. If one of thelocations in the small neighborhood provides better results than thecurrent best location, the current best location is replaced with thebetter location in the small neighborhood, and the new location ischecked in a small neighborhood around it. In this way, the searchprocess using the small neighborhood pattern can iteratively repeat,until the current best location is the best location in the smallneighborhood around it. This might happen in the first iteration orafter multiple iterations.

FIGS. 18 a and 18 b illustrate iterative evaluation of a location in asmall neighborhood, when searching for a BV value or MV value for ablock. In FIG. 18 a , the current best location is shown as a blackcircle. Adjacent locations in a diamond pattern (shown as gray circles)are evaluated. Results for the four adjacent locations, respectively,are compared to results for the current best location (in terms ofdistortion cost, bit rate cost or some combination of distortion costand bit rate cost). In the example of FIGS. 18 a and 18 b , the locationbelow the current best location in FIG. 18 a provides better results andbecomes the current best location. In a next iteration, as shown in FIG.18 b , the adjacent locations in a diamond pattern around the newcurrent best location are evaluated. Alternatively, the smallneighborhood can have another shape.

The encoder can use a threshold to limit the number of iterations in theidentification (1710) stage. The threshold depends on implementation andis, for example, 4 iterations. If a threshold number of iterations isreached, the encoder can perform another search process (e.g., a fullsearch process or a hashing process) to determine the best location forthe prediction.

Returning to FIG. 17 , after identifying the current best location inthe small neighborhood (within the threshold number of iterations), theencoder confirms (1720) the current best location for the predictionthrough iterative evaluation in successively larger neighborhoods aroundthe current best location for the prediction. For example, each of thelarger neighborhoods includes locations in a ring outside the smallneighborhood. The successively larger neighborhoods can growincrementally by one sample, two samples, etc. at each side. Or, thesuccessively larger neighborhoods can be scaled up by some factor. Forexample, the radius of the larger neighborhood is scaled by a factor of2 in each iteration after the first.

If one of the locations in a larger neighborhood provides better resultsthan the current best location, the current best location is replacedwith the better location in the larger neighborhood, and the encoderrestarts the process at the new current best location. Otherwise (noneof the locations in a larger neighborhood provides better results thanthe current best location), the encoder repeats the confirmation processwith the next larger neighborhood, until a threshold number ofiterations of the confirmation (1720) process is reached. The thresholddepends on implementation and is, for example, any of 4 to 7 stages ofchecks for successively larger neighborhoods.

After the largest neighborhood is checked successfully, the encoderterminates the search process. Otherwise (shown as decision 1730), theencoder restarts the process at the new current best location.

FIG. 19 illustrates iterative confirmation of a location in one or morelarger neighborhoods, when searching for a BV value or MV value for ablock. In FIG. 19 , the current best location is shown as a blackcircle. The encoder checks the eight locations at the corners andmidpoints of a square (inner square) around the current best location.The eight locations are shown as gray circles in FIG. 19 . Results forthe eight locations, respectively, are compared to results for thecurrent best location (in terms of distortion cost, bit rate cost orsome combination of distortion cost and bit rate cost). If none of theeight locations in the inner square is better than the current bestlocation, the encoder checks the eight locations (corners and midpoints)of a larger square (outer square in FIG. 19 ). Alternatively, the largerneighborhood can have another shape (e.g., circle of locations), includefewer locations that are evaluated (e.g., just corners) or include morelocations that are evaluated (e.g., locations at perimeter of shape).

A counter for the first threshold (small neighborhood iterations) can bereset every time the identification (1710) stage is started orrestarted. Or, the encoder can reset the counter for the first thresholdonly once, at the beginning of the process (1700). Similarly, thecounter for the second threshold (large neighborhood iterations) can bereset every time the confirmation (1720) stage is started or restarted.Or, the encoder can reset the counter for the second threshold onlyonce, at the beginning of the process (1700). If the counters are resetwithin the process (1700), the encoder can use another constrain tolimit how long the search process continues, ensuring that it terminateswithin a reasonable amount of time.

FIG. 20 is a flowchart illustrating an example technique for searchingfor a BV value or MV value for a block using iterative evaluation of alocation in small neighborhood(s) and iterative confirmation of thelocation in larger neighborhood(s).

The encoder resets (2010) first and second counters. The encoder sets(2020) the current best location and evaluates it (if results for thecurrent best location are not already available from previousevaluation). The encoder then evaluates (2030) adjacent locations in asmall neighborhood around the current best location (if results for theadjacent locations are not already available from previous evaluation).The encoder checks (2040) if a new best location is found (by comparingresults). If so, the encoder checks (2042) if a first threshold isreached using the first counter. If not, the encoder increments thefirst counter (not shown), sets (2010) the current best location to bethe new best location (from among the adjacent locations) and continuesfrom there. In this way, the encoder can iteratively check a smallneighborhood of adjacent locations around a current best location.

If the first threshold is reached, the encoder performs (2080) a fullsearch. Alternatively, the encoder uses another search process such as ahashing process.

If a new best location is not found (at decision (2040)), the encoderevaluates (2050) a ring of locations around the current best location.The encoder checks (2060) if a new best location is found (by comparingresults). If so, the encoder resets (2090) the first and secondcounters, sets (2010) the current best location to be the new bestlocation (from among the adjacent locations), and continues from there.In this way, the encoder restarts the process (2000)

Otherwise (new best location not found at decision (2060)), the encoderchecks (2062) if a second threshold is reached using the second counter.If so, the encoder (successfully) terminates the search process. If not,the encoder increments the second counter (not shown), expands the ring(2070) of locations and evaluates (2050) the (expanded) ring oflocations around the current best location.

In the examples described in this section, BV values and MV valuesindicate integer-sample offsets. Alternatively, BV values and/or MVvalues can indicate fractional-sample offsets. When fractional-sampleoffsets are permitted, the encoder can identify a BV value or MV valuehaving an integer-sample offset as described with reference to FIGS.17-20 . Then, the encoder can identify a BV value or MV value in aneighborhood around the integer-sample BV or MV value (e.g., within asingle-sample offset from the integer-sample BV or MV value).

Alternatively, when fractional-sample offsets are permitted, the encodercan identify a BV value or MV value having a fractional-sample offsetwith the permitted precision at each of the stages described withreference to FIGS. 17-20 (that is, identify a BV value or MV valuehaving a fractional-sample offset in a small neighborhood, identify a BVvalue or MV value having a fractional-sample offset in a largerneighborhood, and so on).

X. CONSTRAINING BV SEARCH RANGE FOR INTRA BC PREDICTION

In some example implementations, an encoder uses a full search range forBV estimation. The entire area of reconstructed sample value is searchedto identify a BV value for a current block. While using a full searchrange can help identify the best BV values to use in intra BCprediction, it can also add to the complexity of BV estimation.

In other example implementations, an encoder limits BV search rangeaccording to one or more constraints. By limiting BV search range, thearea of reconstructed sample values that is referenced by fast memoryaccess for intra BC prediction during encoding and decoding can bereduced, which tends to lower implementation cost.

In the examples in this section, the encoder considers luma samplesvalues of a current luma block when identifying a BV value during BVestimation. The encoder attempts to match luma sample values for thecurrent block to reconstructed luma sample values of previous lumablocks. The resulting BV value is also applied to chroma sample valuesof corresponding chroma blocks, however.

FIG. 21 a illustrates example constraints on search range for BV values.In addition to a current block (2130) of a current frame (2110), FIG. 21a shows a search range defined by two CTBs (2120, 2122). The current CTB(2120) is part of the current CTU and includes the current block (2130).With the CTB (2122) to its left, the current CTB (2120) defines a searchrange within which allowable BVs can be found for the current block(2130). BVs (2142, 2144) reference regions that are outside the searchrange, so those BV values (2142, 2144) are not allowed.

In some example implementations, the search range for BV values for acurrent block is the current CTB and the CTB to its left. For example, aCTB can have size of 64×64, 32×32 or 16×16 sample values, which yields asearch range of 128×64, 64×32 or 32×16 sample values. Only sample valuein the current CTB and left CTB are used for intra BC prediction for thecurrent block. This simplifies encoder implementation by constrainingthe search process. It also simplifies decoder implementation bylimiting the number of sample values that the decoder buffers in fastmemory for intra prediction. (The decoder has adequate buffer capacityto store sample values for two CTBs of the largest possible dimensions,even if a smaller CTU/CTB size is selected.) Another constraint is thatintra prediction cannot reference sample values from another slice ortile. For a current m×n block with a top left position at (x₀, y₀) andCTB(s) each having dimensions CTB_(sizeY)×CTB_(sizeY), an encoder cancheck these constraints for a two-dimensional BV having a horizontalcomponent BV[0] and vertical component BV[1] as follows.

BV[0]≥−((x ₀% CTB _(sizeY))+CTB _(sizeY))

BV[1]≥−(1% CTB _(sizeY))

-   -   The sample values at positions (x₀, y₀), (x₀+BV[0], y₀+BV[1])        and (x₀+BV[0]+m−1, y₀+BV[1]+n−1) shall be in the same slice.    -   The sample values at positions (x₀, y₀), (x₀+BV[0], y₀+BV[1])        and (x₀+BV[0]+m−1, y₀+BV[1]+n−1) shall be in the same tile.

In practice, evaluating candidate BV values in a large, square searchrange (such as an S×S search range, where S is CTB_(sizeY)) or large,rectangular search range (such as a 2S×S search range, where S isCTB_(sizeY)) does not make sense if the best BV values tend to be eitherhorizontally oriented or vertically oriented. Instead, the encoder canuse a smaller search range that still includes likely candidate BVvalues, where the smaller BV search range is horizontally oriented(e.g., with dimensions 2S×¼S or 2S×¾S) or vertically oriented (e.g.,with dimensions ¼S×2S or ¾S×2S). In most scenarios, the encoder checksfewer BV values during BV estimation, but still finds the most suitableBV values.

A BV search range with horizontal bias (or horizontal orientation)includes candidate BV values with a wider range of horizontal BVcomponent values than vertical BV component values. Conversely, a BVsearch range with vertical bias (or vertical orientation) includescandidate BV values with a wider range of vertical BV component valuesthan horizontal BV component values.

The BV search range can also be subject to other constraints. Forexample, the BV search range can be subject to the constraint that anyBV value for the current block reference a region that lies within thecurrent CTB and/or CTB to the left of the current CTB. That is, the BVsearch range fits within the current CTB and CTB to its left. Or, asanother example, the BV search range can be subject to the constraintthat any BV value for the current block reference a region that lieswithin the current CTB and/or CTB above the current CTB. That is, the BVsearch range fits within the current CTB and CTB above it. The BV searchrange can also be constrained to fit within the current picture. In someimplementations, the BV search range is further constrained to fitwithin the current slice and/or current tile.

Aside from a constraint at the far boundary of a BV search range (awayfrom the current block), the BV search range can be constrained at itsnear boundary (close to the current block). For example, in someimplementations, for a CU having size 2N×2N, a PU partition can havesize N×N and 2N×N or N×2N, where each PU has its own BV, or some otherpartition size. The BV of a PU is not allowed to reference other PUregions within the same CU, however. This constraint on BV search rangesomewhat reduces performance relative to allowing PU-level overlap, butallows each PU to have its own BV and allows the PUs within each CU tobe reconstructed in parallel, which may facilitate efficient decoding.

Alternatively, if overlap between a CB and correspondingintra-prediction region is allowed, the encoder can perform overlapprocessing. In this case, the BV search range with horizontal bias orvertical bias can extend into the current CU.

FIG. 21 b shows a first alternative search range (2181) that has ahorizontal bias (horizontal orientation). The search range (2181) hasdimensions of up to 2S×¼S. The search range (2181) may be truncated tofit within the current CTB and CTB to its left, as shown in thecross-hatched and hatched portions of the search range (2181). Or, thesearch range (2181) may be further constrained to not include anyportion in the current CTB (shown as the hatched portion of the searchrange (2181) in FIG. 21 b ).

FIG. 21 c shows a second alternative search range (2182) that has ahorizontal bias. The search range (2182) has dimensions of up to 2S×¾S.The search range (2182) may be truncated to fit within the current CTBand CTB to its left, as shown in the cross-hatched and hatched portionsof the search range (2182). Or, the search range (2182) may be furtherconstrained to not include any portion in the current CTB (shown as thehatched portion of the search range (2182) in FIG. 21 c ).

FIG. 21 d shows a third alternative search range (2183) that has avertical bias (vertical orientation). The search range (2183) hasdimensions of up to ¼S×¾S. The search range (2183) may be truncated tofit within the current CTB and CTB above it, as shown in thecross-hatched and hatched portions of the search range (2183). Or, thesearch range (2183) may be further constrained to not include anyportion in the current CTB (shown as the hatched portion of the searchrange (2183) in FIG. 21 d ).

FIG. 21 e shows a fourth alternative search range (2184) that has avertical bias. The search range (2184) has dimensions of up to ¾S×2S.The search range (2184) may be truncated to fit within the current CTBand CTB above it, as shown in the cross-hatched and hatched portions ofthe search range (2184). Or, the search range (2184) may be furtherconstrained to not include any portion in the current CTB (shown as thehatched portion of the search range (2184) in FIG. 21 d ).

During BV estimation, a video encoder or image encoder can performencoding that includes BV estimation as follows.

The encoder determines a BV for a current block of a picture. The BVindicates a displacement to a region within the picture. The currentblock is in a current CTB having size S. As part of determining the BV,the encoder checks a constraint that the region is within a BV searchrange having a horizontal bias or vertical bias. The encoder performsintra BC prediction for the current block using the BV. The encoder alsoencodes the BV. For example, the encoder performs the BC prediction andencodes the BV as described elsewhere in this application.

More generally, the encoder encodes data for a picture using intra BCprediction. The encoding includes performing BV estimation operationsusing a BV search range with a horizontal or vertical bias. The encoderoutputs the encoded data for the picture.

The BV search range can have a horizontal bias, having dimensions 2S×¼Sor 2S×¾S. Or, more generally, the horizontally-biased BV search rangehas a width between S and 2S, inclusive, and has a height between ¼S and¾S, inclusive. Or, the BV search range can have a vertical bias, havingdimensions ¼S×2S or ¾S×2S. Or, more generally, the vertically-biased BVsearch range has a height between S and 2S, inclusive, and has a widthbetween ¼S and ¾S, inclusive.

The encoder can select the BV search range from among multiple availableBV search ranges. For example, the encoder selects among multiple searchranges having horizontal bias (such as 2S×¼S and 2S×¾S search ranges).Or, the encoder selects among multiple search ranges having verticalbias (such as ¼S×2S and ¾S×2S search ranges). Or, the encoder selectsamong multiple search ranges each having either horizontal bias orvertical bias.

The encoder can select the BV search range based at least in part on BVvalues of one or more previous blocks. For example, the previousblock(s) are in the current picture. Or, the previous block(s) are inone or more previous pictures. Or, the previous blocks are in thecurrent picture and in one or more previous pictures. By considering theBV value(s) of previous block(s), the encoder can identify trends in BVvalues (e.g., that most BV values have a strong horizontal BV componentbut little or no vertical BV component) and select an appropriate BVsearch range. The selection of the BV search range can also depend onother factors (e.g., a user setting).

The BV value(s) for the previous block(s) can be tracked, for example,using a data structure that organizes the BV value(s) as a histogram,with different categories (or “bins”) corresponding to different rangesof BV values and storing a count per category/bin. Thus, the histogramdata structure can provide statistics about the frequency of use ofdifferent BV values. Or, the BV value(s) can be tracked in some otherway. For example, the encoder tracks BV values for blocks of a currentpicture, then evaluates the BV values of previous blocks in aneighborhood around the current block to determine which BV search rangeto use.

Using a smaller BV search range with horizontal or vertical bias may beslightly less efficient (in terms of rate-distortion performance) thanusing a larger, S×S or 2S×S search range. For many encoding scenarios,the reduction in computational complexity of BV estimation justifiesthis penalty.

FIG. 22 shows a technique (2200) for encoding with an intra BCprediction mode, subject to one or more constraints on selection of BVvalues. An encoder such as one described with reference to FIG. 3 orFIGS. 5 a-5 b can perform the technique (2200).

To start, the encoder determines (2210) a BV for a current block of apicture. The current block can be a CB, PB or other block. The BVindicates a displacement to a region within the picture. In determiningthe BV, the encoder checks one or more constraints.

According to one possible constraint, the encoder checks range of samplevalues used for intra BC prediction. The encoder can check that acandidate intra-prediction region is within a range defined by a currentCTB and one or more other CTBs (e.g., CTB to the left of the currentCTB). For example, when the BV has a first component BV[0] and a secondcomponent BV[1], the current block has a top left position at position(x₀, y₀), and each of the CTB(s) has width CTB_(width) and heightCTB_(height), the constraint is satisfied if BV[0]>=−((x₀ %CTB_(width))+CTB_(width)) and BV[1]>=−(y₀ % CTB_(height)). The encodercan similarly check upper limits on values of BV[0] and BV[1] within thesearch range: BV[0]<(CTB_(width)−m−(x0% CTB_(width))) andBV[1]<(CTB_(height)−n−(y0% CTB_(height))). Alternatively, the searchrange includes more or fewer CTBs, or the search range is defined insome other way.

According to another possible constraint, the encoder limits searchingto the current slice and tile (i.e., the current block and region arepart of no more than one slice of the picture and no more than one tileof the picture). The encoder can check that a top left position of thecurrent block, a top left position of a candidate intra-predictionregion and a bottom right position of the candidate intra-predictionregion are part of a single slice and single tile. For example, theconstraint is satisfied if (x₀, y₀), (x₀+BV[0], y₀+BV[1]) and(x₀+BV[0]+m−1, y₀+BV[1]+n−1) are part of a single slice and single tile.

Alternatively, the encoder checks other and/or additional constraints.

The encoder performs (2220) intra BC prediction for the current blockusing the BV. For example, the encoder performs intra BC prediction forthe entire current block. Or, the encoder performs intra BC predictionfor multiple blocks associated with the current block (e.g., formultiple TBs on a TB-by-TB basis, where the TBs are associated with acurrent PB that has the BV).

The encoder encodes (2230) the BV. The encoder can repeat the technique(2200) for another intra BC prediction mode block.

For intra BC prediction, the encoder and decoder use reconstructedsample values. Unreconstructed sample values might be present as partsof a picture that have not been encoded and reconstructed yet. To avoidusing unreconstructed sample values for intra BC prediction, the encodercan set constraints on allowable values of BV such that only actual,previously reconstructed sample values are used for intra BC predictionaccording to a BV.

In some example implementations, the encoder checks a BV value byconsidering the z-scan orders of the current block and the block thatcontains the bottom right position of the candidate intra-predictionregion. More specifically, the encoder checks that the z-scan order ofthe block containing the position (x₀+BV[0]+m−1, y₀+BV[1]+n−1) issmaller than z-scan order of the block containing (x₀, y₀). If so, theblock that contains the bottom right position of the intra-predictionregion has been previously reconstructed (and hence so has the rest ofthe intra-prediction region). The BV also satisfies at least one of theconditions BV[0]+m≤0 and BV[1]+n≤0, ensuring that the intra-predictionregion does not overlap the current block.

The z-scan order follows a sequentially specified ordering of blocksthat partition a picture. FIG. 23 shows example z-scan order (2300) fora current block (2330) and blocks that might include the bottom rightposition of an intra-prediction region for a candidate BV. The currentblock (2330) can be a CB, PB or other block. The z-scan orders aregenerally assigned to blocks sequentially from left-to-right in a row,repeating in successive rows from top-to-bottom. When a block is split,z-scan orders are assigned within the split block, recursively. Forimplementations of encoding/decoding for the HEVC standard, the z-scanorder proceeds CTB-to-CTB by a CTB raster scan pattern (left-to-right ina CTB row, repeating in successive CTB rows from top-to-bottom). If aCTB is split, the z-scan order follows a raster scan pattern for CBs ofa quadtree within the split CTB. And, if a CB is split (e.g., intomultiple CBs, or into multiple PBs), the z-scan order follows a rasterscan pattern for blocks within the split CB.

Alternatively, when intra BC prediction can be performed on a TB-by-TBbasis, the encoder and decoder can check for possible overlap between anintra-prediction region and a current block (TB), then use the resultsof the check to decide whether the current TB should be split intosmaller TBs for application of intra BC prediction operations. Suppose acurrent TB has a size of m×n, where m and n can be equal to each otheror can have different values. If BV[0]>−m and BV[1]>−n, theintra-prediction region overlaps the current m×n TB, which isproblematic unless the current m×n TB is split into smaller TBs forapplication of intra BC prediction operations. Thus, if BV[0]>−m andBV[1]>−n, the encoder and decoder split the current TB into smaller TBs.The same condition is checked (e.g., checked recursively) for thesmaller TBs, which may be further split if BV[0]>−m and BV[1]>−n evenfor the smaller values of m and n after splitting.

For example, suppose the BV for a PB is (−9, −5), and the current TB isa 32×32 block. The encoder and decoder determine that −9>−32 and −5>−32,indicating that the intra-prediction region (whose top left corner isdisplaced −9, −5) would overlap the current 32×32 TB. The encoder anddecoder split the 32×32 TB into four 16×16 TBs. For each of the 16×16TBs, the encoder and decoder determine that −9>−16 and −5>−16,indicating that the intra-prediction region (whose top left corner isdisplaced −9, −5) would overlap the current 16×16 TB. The encoder anddecoder split each 16×16 TB, in succession, into four 8×8 TBs. For an8×8 TB, the BV of (−9, −5) is not problematic, so the 8×8 TB is notforced to be further split.

In this scenario, when a TB is split due to a BV value and size of theTB, the encoder can skip signaling of the flag value that wouldotherwise signal whether to split the current TB into smaller TBs. Thebitstream of encoded data lacks the flag value directing the decoder tosplit the current TB into smaller TBs. Instead, the decoder can inferthat a TB should be split due to a BV value and the size of the TB. Thiscan save bits that would otherwise be spent signaling information aboutsplitting TBs.

As part of BV estimation, the encoder can use any of several approaches.The encoder can use a full search, evaluating every candidate BV valueallowed in a search range. Or, the encoder can use a partial search,evaluating only some of the candidate BV values allowed in a searchrange. For example, the encoder can start a partial search at thepredicted BV value for a current block (e.g., predicted based on BVvalues of one or more neighboring blocks). After evaluating thecandidate BV value at the starting position for the partial search, theencoder can evaluate one or more other candidate BV values at increasingdistances from the starting position (e.g., according to a spiral searchpattern or some other pattern). Or, the encoder can use a search patternas described in the previous section. When evaluating a given candidateBV value, the encoder can compare all sample values in theintra-prediction region and current block. Or, the encoder can evaluatea subset of the sample values (that is, sub-sample which values areevaluated). When comparing sample values between the intra-predictionregion and current block to determine a distortion cost, the encoder cancompute mean square error, sum of squared differences (“SSD”), sum ofabsolute differences (“SAD”), or some other measure of distortion. Theencoder can also determine bit rate costs associated with encoding ofthe candidate BV value.

XI. ALTERNATIVES AND VARIATIONS

In many of the examples described herein, intra BC prediction and motioncompensation are implemented in separate components or processes, and BVestimation and motion estimation are implemented in separate componentsor processes. Alternatively, intra BC prediction can be implemented as aspecial case of motion compensation, and BV estimation can beimplemented as a special case of motion estimation, for which thecurrent picture is used as a reference picture. In such implementations,a BV value can be signaled as an MV value but used for intra BCprediction (within the current picture) rather than inter-pictureprediction. As the term is used herein, “intra BC prediction” indicatesprediction within a current picture, whether that prediction is providedusing an intra-picture prediction module, a motion compensation module,or some other module. Similarly, a BV value can be represented using anMV value or using a distinct type of parameter or syntax element, and BVestimation can be provided using an intra-picture estimation module,motion estimation module or some other module.

In view of the many possible embodiments to which the principles of thedisclosed invention may be applied, it should be recognized that theillustrated embodiments are only preferred examples of the invention andshould not be taken as limiting the scope of the invention. Rather, thescope of the invention is defined by the following claims. We thereforeclaim as our invention all that comes within the scope and spirit ofthese claims.

We claim:
 1. One or more non-transitory computer-readable media havingstored thereon computer-executable instructions for causing one or moreprocessing units, when programmed thereby, to perform operationscomprising: receiving a frame of a video sequence; splitting the frameinto multiple 2N×2N blocks, the multiple 2N×2N blocks including acurrent 2N×2N block of the frame; encoding the current 2N×2N block ofthe frame to produce encoded data, the current 2N×2N block being withina given slice of the frame and being within a given tile of the frame,the current 2N×2N block having dimensions of 64×64, 32×32, or 16×16,including: constraining values of vectors for intra block copy (“BC”)prediction to have integer-sample precision and to indicate referenceregions that are within the given slice, within the given tile, and notoverlapping the current 2N×2N block within the frame; asymmetricallypartitioning the current 2N×2N block into first and second partitionsfor intra BC prediction by splitting the current 2N×2N blockhorizontally or vertically into the first and second partitions, thefirst and second partitions having dimensions of: (a) 64×16 and 64×48,respectively, or 16×64 and 48×64, respectively, if the current 2N×2Nblock has dimensions of 64×64; (b) 32×8 and 32×24, respectively, or 8×32and 24×32, respectively, if the current 2N×2N block has dimensions of32×32; and (c) 16×4 and 16×12, respectively, or 4×16 and 12×16,respectively, if the current 2N×2N block has dimensions of 16×16;performing intra BC prediction for the first partition; and performingintra BC prediction for the second partition; and outputting the encodeddata as part of a bitstream.
 2. The one or more computer-readable mediaof claim 1, wherein the values of vectors for intra BC prediction arefurther constrained to indicate displacements to reference regionswithin a search range having a horizontal bias or a vertical bias,wherein, for the horizontal bias, the search range includes candidatevector values having a wider range of horizontal component values thanvertical component values, and wherein, for the vertical bias, thesearch range includes candidate vector values having a wider range ofvertical component values than horizontal component values.
 3. The oneor more computer-readable media of claim 1, wherein the encoding furtherincludes, for the current 2N×2N block: determining a first vector thatindicates a first displacement to a first reference region of the frame;determining a second vector that indicates a second displacement to asecond reference region of the frame; as part of the performing intra BCprediction for the first partition, using the first vector to determinepredicted sample values for the first partition from previouslyreconstructed sample values in the first reference region of the frame;and as part of the performing intra BC prediction for the secondpartition, using the second vector to determine predicted sample valuesfor the second partition from previously reconstructed sample values inthe second reference region of the frame.
 4. The one or morecomputer-readable media of claim 3, wherein the determining the firstvector includes identifying the first vector subject to a constraintthat the first reference region is within a search range having ahorizontal bias or a vertical bias, wherein, for the horizontal bias,the search range includes candidate vector values having a wider rangeof horizontal component values than vertical component values, andwherein, for the vertical bias, the search range includes candidatevector values having a wider range of vertical component values thanhorizontal component values.
 5. The one or more computer-readable mediaof claim 3, wherein the determining the first vector includes:identifying a current best location for prediction through iterativeevaluation in a small neighborhood around the current best location,wherein the small neighborhood includes locations that are immediatelyadjacent horizontally or vertically to the current best location; andconfirming the current best location through iterative evaluation insuccessively larger neighborhoods around the current best location,wherein each of the larger neighborhoods includes locations in a ringoutside the small neighborhood.
 6. The one or more computer-readablemedia of claim 1, wherein the encoding further includes performing intraBC prediction for another 2N×2N block that is symmetrically partitionedfor the intra BC prediction, and wherein the other 2N×2N block ispartitioned into (1) two 2N×N blocks, (2) two N×2N blocks, or (3) fourN×N blocks, each of which can be further partitioned into two N×N/2blocks, two N/2×N blocks or four N/2×N/2 blocks.
 7. The one or morecomputer-readable media of claim 1, wherein the current 2N×2N block issplit horizontally, and the first and second partitions are top andbottom partitions, respectively.
 8. The one or more computer-readablemedia of claim 1, wherein the current 2N×2N block is split vertically,and the first and second partitions are left and right partitions,respectively.
 9. The one or more computer-readable media of claim 1,wherein the asymmetrically partitioning includes identifying how topartition the current 2N×2N block using a bottom-up approach thatincludes: checking a subset of modes per N×N block of the current 2N×2Nblock; caching vector values for the respective N×N blocks of thecurrent 2N×2N block; checking a subset of modes with a 2N-dimension forthe current 2N×2N block, including using the cached vector values;selecting a best mode with a 2N-dimension for the current 2N×2N block;and selecting between the best mode with a 2N-dimension for the current2N×2N block and best modes for the respective N×N blocks of the current2N×2N block.
 10. The one or more computer-readable media of claim 1,wherein the asymmetrically partitioning includes identifying how topartition the current 2N×2N block using a top-down approach thatincludes: checking modes with a 2N-dimension for the current 2N×2Nblock; selecting a best mode with a 2N-dimension for the current 2N×2Nblock; for each N×N block of the current 2N×2N block, checking modes forthe N×N block; selecting a best combination of modes for the respectiveN×N blocks of the current 2N×2N block; and selecting between the bestmode with a 2N-dimension for the current 2N×2N block and the bestcombination of modes for the respective N×N blocks of the current 2N×2Nblock.
 11. In a computer system that implements video decoder, a methodcomprising: receiving encoded data as part of a bitstream; and decodingthe encoded data to reconstruct a frame of a video sequence, includingperforming intra block copy (“BC”) prediction for first and secondpartitions of a current 2N×2N block of the frame, the current 2N×2Nblock being within a given slice of the frame and being within a giventile of the frame, and values of vectors for the intra BC predictionbeing constrained to have integer-sample precision and to indicatereference regions that are within the given slice, within the giventile, and not overlapping the current 2N×2N block within the frame, thecurrent 2N×2N block having been asymmetrically partitioned for the intraBC prediction, the current 2N×2N block having dimensions of 64×64,32×32, or 16×16, the current 2N×2N block having been split horizontallyor vertically into the first and second partitions, the first and secondpartitions having dimensions of: (a) 64×16 and 64×48, respectively, or16×64 and 48×64, respectively, if the current 2N×2N block has dimensionsof 64×64; (b) 32×8 and 32×24, respectively, or 8×32 and 24×32,respectively, if the current 2N×2N block has dimensions of 32×32; and(c) 16×4 and 16×12, respectively, or 4×16 and 12×16, respectively, ifthe current 2N×2N block has dimensions of 16×16.
 12. The method of claim11, wherein the decoding further includes performing intra BC predictionfor another 2N×2N block that has been symmetrically partitioned for theintra BC prediction, and wherein the other 2N×2N block has beenpartitioned into (1) two 2N×N blocks, (2) two N×2N blocks, or (3) fourN×N blocks, each of which can be further partitioned into two N×N/2blocks, two N/2×N blocks or four N/2×N/2 blocks.
 13. The method of claim11, wherein the decoding includes, for the current 2N×2N block:reconstructing a first vector that indicates a first displacement to afirst reference region of the frame; reconstructing a second vector thatindicates a second displacement to a second reference region of theframe; as part of the performing intra BC prediction for the firstpartition, using the first vector to determine predicted sample valuesfor the first partition from previously reconstructed sample values inthe first reference region of the frame; and as part of the performingintra BC prediction for the second partition, using the second vector todetermine predicted sample values for the second partition frompreviously reconstructed sample values in the second reference region ofthe frame.
 14. The method of claim 11, wherein the current 2N×2N blockhas been split horizontally, and the first and second partitions are topand bottom partitions, respectively.
 15. The method of claim 11, whereinthe current 2N×2N block has been split vertically, and the first andsecond partitions are left and right partitions, respectively.
 16. Oneor more non-transitory computer-readable media having stored thereonencoded data as part of a bitstream, the encoded data being organized tofacilitate decoding, with a computer system that implements a videodecoder, by operations comprising: receiving the encoded data; anddecoding the encoded data to reconstruct a frame of a video sequence,including performing intra block copy (“BC”) prediction for first andsecond partitions of a current 2N×2N block of the frame, the current2N×2N block being within a given slice of the frame and being within agiven tile of the frame, and values of vectors for the intra BCprediction being constrained to have integer-sample precision and toindicate reference regions that are within the given slice, within thegiven tile, and not overlapping the current 2N×2N block within theframe, the current 2N×2N block having been asymmetrically partitionedfor the intra BC prediction, the current 2N×2N block having dimensionsof 64×64, 32×32, or 16×16, the current 2N×2N block having been splithorizontally or vertically into the first and second partitions, thefirst and second partitions having dimensions of: (a) 64×16 and 64×48,respectively, or 16×64 and 48×64, respectively, if the current 2N×2Nblock has dimensions of 64×64; (b) 32×8 and 32×24, respectively, or 8×32and 24×32, respectively, if the current 2N×2N block has dimensions of32×32; and (c) 16×4 and 16×12, respectively, or 4×16 and 12×16,respectively, if the current 2N×2N block has dimensions of 16×16. 17.The one or more computer-readable media of claim 16, wherein thedecoding further includes performing intra BC prediction for another2N×2N block that has been symmetrically partitioned for the intra BCprediction, and wherein the other 2N×2N block has been partitioned into(1) two 2N×N blocks, (2) two N×2N blocks, or (3) four N×N blocks, eachof which can be further partitioned into two N×N/2 blocks, two N/2×Nblocks or four N/2×N/2 blocks.
 18. The one or more computer-readablemedia of claim 16, wherein the decoding includes, for the current 2N×2Nblock: reconstructing a first vector that indicates a first displacementto a first reference region of the frame; reconstructing a second vectorthat indicates a second displacement to a second reference region of theframe; as part of the performing intra BC prediction for the firstpartition, using the first vector to determine predicted sample valuesfor the first partition from previously reconstructed sample values inthe first reference region of the frame; and as part of the performingintra BC prediction for the second partition, using the second vector todetermine predicted sample values for the second partition frompreviously reconstructed sample values in the second reference region ofthe frame.
 19. The one or more computer-readable media of claim 16,wherein the current 2N×2N block has been split horizontally, and thefirst and second partitions are top and bottom partitions, respectively.20. The one or more computer-readable media of claim 16, wherein thecurrent 2N×2N block has been split vertically, and the first and secondpartitions are left and right partitions, respectively.